Ahead of the NSW 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 sliver of green vegetation would stretch from both sides of the Hawkesbury River northwest of Sydney until the 120-kilometre-long river meets the sea.
The group also wants underused sports grounds converted into green spaces and says the government needs to protect open spaces from development.
It comes as large parts of western Sydney are set for major new housing developments.
Planning Minister Anthony Roberts on Monday announced 70,000 new homes could be created through rezoning projects, on top of plans for 19,000 new homes in the Macarthur Region where the state government is taking charge of planning proposals usually managed by local councils.
Mr Roberts said the construction, part of the government's $2.8 billion housing strategy, would happen alongside the creation of new koala corridors.
But without an effective and sustainable green spaces and tree retention policy, the government had already overseen the loss of "thousands of mature trees", the centre's director Jeff Angel said.
The centre suggested a dedicated legal office be established to challenge land rezonings and sales that are not in the public interest.
NSW Labor, if elected in March, plans to restore the environment by developing relationships with landholders and local communities, the party's planning and public spaces spokesman Paul Scully said.
"Our planning system and environmental laws must work to protect the environment as well as build the infrastructure that NSW needs," he told AAP.
Labor has also committed to completing the Wolli Creek Regional Park project, first launched in 1998, by acquiring outstanding land pockets, and protecting bushland in Sydney's south.
Greens MP Cate Faehrmann said Sydney's green spaces had been fractured by urban sprawl and overdevelopment.
What is left urgently needs protecting, she said.
"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 southwest fringe," she told AAP.
It's not just bad for the environment but also for the families that end up living there, Ms Faehrmann said.
AAP has contacted Mr Roberts for comment.
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