Below the surface
Transport for NSW has partnered with experts from the University of NSW and local Gamay Rangers to protect endangered species Posidonia australis seagrass and White’s Seahorse.
What is Posidonia australis?
Posidonia australis (pronounced ‘Pos-ee-doe-nee-ah, Os-trah-lees’) is an important seagrass that can be found in coastlines along the southern half of Australia, including in 17 estuaries along the east coast of NSW.
In NSW, Posidonia likes to live in shallow, sheltered bays, estuaries, and coastal lakes. Its leaves are bright green and strap-like (about 1 cm wide) and can grow to over 80 cm long. As much as 90 percent of the mass of the plant may be in the underground rhizome/roots. This is part of what makes seagrasses different to seaweeds; seagrasses are vascular plants that have roots, leaves, flowers and fruits. In contrast, seaweeds are a type of algae, which are evolutionarily simpler organisms and lack a vascular system (the internal transport system for nutrients and fluids).
There are about 70 species of seagrass world-wide. Seagrasses are one of the world’s oldest flowering plants, and have adapted to live fully submerged underwater. You may have heard seagrasses referred to as the ‘lungs of the ocean’, that’s because as they photosynthesise, they produce oxygen while also capturing and storing large amounts of carbon. Seagrasses also help to prevent erosion and provide shelter for many aquatic animals, including White’s Seahorse in the Sydney region.
In Botany Bay, you can find seagrass species Zostera (eelgrass), Halophila (paddleweed) and Posidonia.
Increased human activity in coastal areas has had a major impact on Posidonia populations, although natural causes, such as storm events, can also lead to declines.
Posidonia is currently listed as endangered in six NSW estuaries, including Botany Bay.
Some seagrasses, like Halophila and Zostera, grow fast and can bounce back after impacts relatively quickly. In contrast, Posidonia grows slowly and is particularly sensitive to disturbances due to ongoing declines, which make the species less able to bounce back. Transport is providing a helping hand to restore and improve Posidonia habitat as part of the wharves project.
To learn more about Posidonia, visit the Department of Primary Industries and Regional Development and search ‘Posidonia australis’.
Seagrass superpowers
- Coastal Aboriginal communities have long used seagrasses in multiple ways, including for clothing, basket weaving, medicinal purposes and in cultural ceremonies
- Seagrass meadows can store up to four times more carbon than terrestrial rainforests
- They provide habitat and nurseries for many species of crab and fish
- Seagrass improves water quality and helps reduce coastal erosion
Our plan to protect these important species
While the Kamay wharves were designed to avoid as many impacts as possible, construction and operation activities have some impact on seagrass, including Posidonia at the Kurnell wharf site.
We’ve developed a plan called the Marine Biodiversity Offset Strategy (MBOS) which sets out how we are managing our impact through relocating ofPosidonia away from the project impact area, and importantly, creating new areas of Posidonia growth in Botany Bay.
As part of this plan, we have also provided new habitat for Syngnathids (the family of fish that include seahorses, seadragons, pipehorses or pipefish).
We have an ambitious seagrass restoration goal, which is why we are committed to regular, long-term monitoring to ensure that we meet our objectives.
The important data collected will also help to shape future Posidonia restoration projects in Australia.
An MBOS Implementation Reference Panel is overseeing the implementation of the MBOS which will be updated as required. Visit our website to view the MBOS.
UNSW
Steps to Posidonia seagrass restoration
Planning
During 2021 and 2022, marine ecologists surveyed and mapped the project impact area to understand Posidonia patterns of growth. They also surveyed the nearby Posidonia meadow at Kurnell to carefully select the best sites to rehabilitate by replanting them with Posidonia. Some of these sites include areas where we can heal existing ‘scars’ or damaged parts of the meadow.
Harvesting and replanting
Harvesting Posidonia from the project impact area at Kurnell took place in 2023.
It involved scientific divers gently removing plants from the sediment, being careful to protect the rhizomes/root area. The plants were then replanted in the nearby rehabilitation sites.
In some sites, biodegradable jute matting material was used to help anchor the plants in the seafloor.
Around 13,000 seagrass shoots were relocated and the area mapped and surveyed to assist with future monitoring.
Replanting naturally detached Posidonia fragments
During intense weather, Posidonia plants naturally break away from meadows and wash ashore. The Gamay Rangers and UNSW Scientists collect suitable naturally detached fragments from beaches around Botany Bay and store them in specialised holding tanks. The fragments are then replanted in rehabilitation sites in Kurnell.
Monitoring
Long-term monitoring is a vital part of achieving our MBOS objectives. It allows us to adapt our work where necessary to get the best outcome.
UNSW Scientists are monitoring factors such as the increase in area, density, condition, and function of the seagrass rehabilitation areas, and comparing it with existing ‘reference’ sites, to gauge the progress of the rehabilitation efforts over time.
The latest technology, including the underwater Hullbot robot, is being used to provide 3D model images to show visual changes in the rehabilitation sites over time.
A ten-year monitoring program is underway, with additional post-storm monitoring to take place when needed to assess any naturally occurring impacts. Monitoring reports are available on the project documents webpage.
No anchoring in Posidonia protection zone
Seagrasses located in shallow areas such as Botany Bay, Kurnell, are vulnerable to damage from boat anchors and propellers which can uproot the plants. To help protect the seagrass rehabilitation sites at Kurnell, a ‘no-anchoring’ zone is in place. Please follow signs and yellow marker buoys. We also encourage boaters to lift propellers when boating in shallow depths around seagrass.
Map of seagrass protection zone
Homes for seahorses in Kamay Botany Bay
As part of our MBOS, we have provided artificial habitat for endangered White’s Seahorse, called seahorse hotels.
Hippocampus whitei (or White’s Seahorse), is a medium-sized seahorse species which can be found on the east coast of Australia, ranging from Hervey Bay in South-East Queensland to Sussex Inlet in the mid-New South Wales coast.
White’s Seahorse are known to live in seagrasses, including Posidonia australis. Several have been spotted during project dive inspections.
We collaborated with NSW Department of Primary Industries and Regional Development Fisheries, seahorse experts from the Sydney Institute for Marine Science (SIMS), Gamay Rangers and the MBOS Implementation Reference Panel to develop an artificial habitat implementation plan, which includes a five-year monitoring program to assess its success.
What is a seahorse hotel?
The hotels are purpose-built metal units that start as artificial habitats, but are rapidly encrusted by algae, sponges and corals once placed in the marine environment (this is called biofouling).
Over time, the marine growth forms a new scaffolding and structure while the metal corrodes leaving a new natural habitat behind and perfect homes for seahorses.
This is not the first-time seahorse hotels have been used – you can read more about how these simple, yet effective creations are being used in NSW on these websites:
- Department of Primary Industries and Regional Development – seahorse hotels and captive breeding program
- Sydney Institute of Marine Science – The Sydney Seahorse Project
- UNSW – Operation Posidonia seahorse hotels
UNSW
Where are the seahorse hotels located?
In July 2025, we installed 60 seahorse hotels within Botany Bay at:
- Frenchmans Bay and Congwong Bay on the La Perouse side
- Inscription Point and Silver Beach on the Kurnell side.
The site selection was based on expert advice from Sydney Institute of Marine Science (SIMS) and consultation with multiple stakeholders including DPIRD Fisheries, Port Authority NSW, and the MBOS Panel.
Some of the key considerations when choosing the sites included:
- if White’s Seahorse are known to be in the area
- nearby presence of seahorse habitat including seagrass or sponges
- suitable marine conditions such as water depth, swell and exposure
- local cultural knowledge provided by the Gamay Rangers.
And in early 2026, SIMS plans to release captive-bred White’s Seahorses into the hotels.
Ripple effects
In addition to creating a net gain of White’s Seahorse habitat, there will be other indirect benefits of this work including:
- providing suitable release sites for White’s Seahorse captive breeding program
- providing habitat that benefits other marine life including other Syngnathiformes
- contributing to research which addresses knowledge gaps for White’s Seahorse, including an improved understanding of the population of White’s Seahorse locally
- providing an opportunity for community members to contribute information about seahorse sightings via existing platforms that help scientific research
- collaborating on the monitoring program with Gamay Rangers to support ongoing connections to Sea Country and building on skills in associated marine ecology field experience.
Tracking success
Marine specialists will help us to monitor factors such as:
- the amount of growth on the hotels over time
- observations of wild and released White’s Seahorse using the hotels to learn more about the health and size of the local population
- use by other seahorse and marine animals.
Monitoring started in August 2025 as part of a five-year program, with Annual reports published on the project webpage.