Ahead of the state election in March, the Total Environment Centre on Wednesday called for protections limiting major development, including enforcing a new koala green belt around Sydney.
The 400-metre wide belt of green vegetation would stretch from both sides of the Hawkesbury River northwest of Sydney, until the river meets the sea.
The group also wants underused sports grounds to be converted into green spaces, and says the government needs to protect open spaces from development.
It suggested the establishment of an Open Spaces Defenders Office to challenge land rezonings and sales that are not in the public interest.
Total Environment Centre Director Jeff Angel says without an effective and sustainable green spaces and tree retention policy "the government has overseen the loss of thousands of mature trees".
It comes as large parts of western Sydney are set for major new housing developments, with Planning Minister Anthony Roberts on Monday announcing that 70,000 new homes could be created through rezoning projects.
That followed another project announcement earlier this month where the government announced it would take responsibility of three planning proposals in the Macarthur Region - usually managed by local councils.
Mr Roberts said at the time the three projects could lead to the construction of 19,000 homes, which would happen in conjunction with the building of new koala corridors.
The two announcements are part of the government's $2.8 billion housing strategy.
Greens MP Cate Faehrmann said Sydney's green spaces had been fractured by urban sprawl and overdevelopment, and what is left urgently needs protecting.
"Sydney has some of the healthiest koala populations in the state but they're being cut off from the green corridors they rely on to survive by new suburbs in Sydney's south-west fringe," she told AAP.
"It's death by a thousand cuts for Sydney's public green spaces because successive governments haven't seen it as a priority to protect them," she said.
"We can't just keep allowing urban sprawl on the fringes of Sydney at the expense of green spaces and biodiversity.
"It's not just bad for the environment, it's bad for the families that end up living there too."
AAP has contacted Mr Roberts for comment.
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