The hill that dominates the area was created out of rubbish piled up from the Alexandria Landfill site, and capped with a layer of crushed sandstone.
Transport for NSW said final work on the six-hectare park was under way, which included repairing damage caused by storms.
“Later this year, we’ll open six hectares of new landscaped parkland at the interchange, including public art installations and new shared paths. More than 650,000 plants, shrubs and trees have been planted,” it said.
The agency has not put a firm date on when the park will be opened. The parkland is a condition of approval for the second stage of the 33-kilometre WestConnex motorway.
Charlie Pierce, a waste management consultant who wrote the first version of the NSW landfill guidelines, said regeneration of the hill’s slopes should have been completed earlier.
“It means more rain getting into the dirt and possibly causing greater issues in leachate management,” said Mr Pierce, who lives in nearby Newtown and has advised the WestConnex Action Group in the past. “There is erosion there. It is not anywhere near ready.”
Inner West independent councillor Pauline Lockie, a critic of WestConnex, said she had long harboured “huge reservations” that usable parkland would be delivered because it had been built on top of a “very toxic landfill”.
“The big hill appears to be eroding or collapsing every time it rains,” she said. “Given we don’t have an opening date for the park, it makes me wonder how they have capped all of that waste.”
WestConnex, which is majority owned by a Transurban-led consortium, said a small area of the mound had been impacted by recent heavy rains due to landscaping not yet being established.
“A biodegradable binding agent had been applied to some areas to prevent potential slippages while landscaping was establishing,” it said.
The toll road operator said mesh had been laid at various places around the interchange as an erosion and sediment control measure, and to help with establishing seeds and landscaping.
Under early plans, an extra 2.5 hectares of parkland towards the north-eastern end of the interchange was to be finished by 2023, and to include a soccer field, options for multi-purpose sports courts, and paths linked to Sydney Park via a new pedestrian bridge.
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Matt O’Sullivan is City Editor at The Sydney Morning Herald.