Waterbirds at the Western Treatment Plant

Encouraging trends

Red‑necked Stint, Sharp‑tailed Sandpiper, Pied Stilt and Australian Pied Oystercatcher have all increased over the long term at the WTP. Red‑necked Avocet numbers are currently stable after earlier growth.

Serious declines

Some shorebird species have declined throughout the East-Asian Australasian Flyway because of habitat destruction at stopover sites in Asia. This has impacted numbers of these shorebird species at the WTP. For example, Eastern Curlew now appears to be locally extinct at the WTP.

Curlew Sandpiper numbers have been stable for the last ten years or so, but they have not returned to their 1980s levels after decades of decline at the WTP and across the flyway.

Dynamic use of habitats

Preferred foraging sites change over time. During 2024-25, most shorebirds foraged on the tidal flats of The Spit Nature Conservation Reserve at low tide. At high tide, WTP shorebirds seem to prefer conservation ponds at the WTP, where they can roost, or do some top-up feeding if they didn’t find enough food at low tide.

At times of limited foraging opportunity in the conservation ponds, many WTP shorebirds roost on the barrier islands of the Spits Nature Reserve, or fly 10 to 15 km to the flooded salt-pans in the Avalon Coastal Reserve.

Climate-driven booms and busts

Waterfowl numbers at the WTP rise and fall with inland flooding and drought. After several wet La Niña years drew birds inland, 2024–25 saw higher duck numbers again at the WTP during a relatively dry year.

Long-term declines

Australasian Shoveler, Blue-billed Duck and Musk Duck. The causes of the declines are unknown but they are suspected to reflect Victoria-wide declines in these species, all of which are now listed as Vulnerable in Victoria.

Unexpected winter peaks

Pacific Black Duck and Grey Teal, usually most abundant in summer, have shown unusually high winter counts in recent years, suggesting changing movement or habitat patterns.

Ibis are still declining overall

Despite numbers during the 2024-2025 season being the highest in several years, yearly monitoring data since 2001 confirms a continuing long-term decline in Straw-necked Ibis using the WTP paddocks. This is consistent with the general population trends of large wading birds across eastern Australia. Irrigated grass is a key foraging habitat, and active management of a subset of paddocks is helping maximise habitat availability.

Valuable paddocks

Active management of conservation paddocks in the Terrestrial Margin area has increased the proportion of ibis foraging within this buffer zone over the long term. This demonstrates that targeted habitat management can shift bird use into conservation areas.

Brolga and other large waders

The WTP regularly supports notable numbers of large wading birds. At least 13 Brolga—listed as Endangered in Victoria—were recorded in a single count, and pairs have attempted breeding in the south‑west of the site.

Freshwater terns

The WTP holds spectacular numbers of Whiskered Terns in spring and early summer, often exceeding 5000 birds. Whiskered Tern numbers increased at the WTP in 2024–25, possibly due to the drying of inland wetlands after major flooding. Long-term models show no clear trend for Whiskered or White‑winged Black Terns, but highlight their sensitivity to inland conditions.

Fairy and Little Terns

The WTP-Avalon coast is now recognised as a major non‑breeding area for Fairy Tern. Autumn–winter counts can exceed 100 birds, accounting for up to two‑thirds of the estimated Victorian population. Little Tern numbers peak in summer, with a mix of Asian migrants and birds from the small south‑eastern Australian breeding population.

Cormorant breeding

The 25W Lagoon colony remains a key breeding site for Pied Cormorant. Recent monitoring shows consistent nesting between January and April, with occasional strong mid‑year breeding events that require a longer time series to understand better.

Rising sea levels will impact shorebirds by reducing the area and shortening the duration of tidal flat exposure. To estimate this impact, we used a combination of foraging theory and empirical data on shorebirds and their prey, collected at the WTP over the past 20 years.

Key findings

• For shorebirds, the reduced duration of tidal flat exposure will have more of an impact than the decreased area of tidal flats.
• There is some capacity at the WTP for reduced tidal flat exposure time without loss of shorebirds. This is because of the high prey abundance on the tidal flats, and top-up feeding can be done in conservation ponds at high tide.
• Reduced foraging intake because of rising sea-levels is likely to first impact shorebirds during their period of pre-migratory mass gain (Feb-April), when their food requirements are highest. Prolonged periods of low air pressure and southerly winds are also challenging for migratory shorebirds at the WTP, as they result in limited tidal flat exposure.

We assessed how much each waterfowl species relies on the seaward ponds—prey‑rich, late‑stage treatment areas—to understand the potential impacts of losing these habitats to sea‑level rise.

Key findings:

• Losing the seaward row of treatment ponds to sea‑level rise would reduce the WTP’s pond surface area by 11%, displacing an estimated 4,000–14,000 waterbirds in summer that would need suitable habitat elsewhere.
• Carnivorous diving birds such as Musk Ducks are likely to be most affected by the loss of seaward ponds, because they depend on this deep, prey‑rich habitat that isn’t available elsewhere at the WTP.

Our research is supporting Melbourne Water’s Foreshore Adaptation Plan, allowing them to consider options such as coastal retreat, engineered foreshore works, increasing the carrying capacity and strategic creation or reconfiguration of ponds and tidal flats.