Poison Gully stretches from Stirk Park in Kalamunda down to Dundas Road, where the proposed Forrestfield Airport Link station will be situated.
Volunteers have been restoring the bank and bushland around the waterway for 16 years.
Nature Reserves Preservation Group president Tony Fowler said the bush was home to threatened ecological communities, a Bush Forever site and black cockatoo habitat.
�This is yet another case of remnant vegetation, including Bush Forever sites, under threat of damage from development,� he said.
�They never seem to acknowledge the need for larger buffers on creek lines and the effect significant buffers have on the quality of the creek line.�
In the event of any damage to the bushland and waterway,
Mr Fowler said the problem with an offset was that the pocket of bush volunteers had worked so hard to protect and rejuvenate would be lost.
The EPA report acknowledges the project directly affects areas of bush described as �banksia woodlands over species-rich dense shrublands�; a threatened ecological community.
EPA chairman Paul Vogel said potential environmental impacts on flora and vegetation at the Forrestfield station were identified as a key factor.
�The proponent has adequately demonstrated why it cannot avoid this community and is committed to providing an offset to mitigate the impacts,� he said.
�As a result of EPA�s assessment of these potential impacts, we were able to conclude that the proposal could be managed to meet our environmental objectives, subject to the implementation of several conditions.�
A separate WA Planning Commission report released in June flags land east of the proposed train station, following Poison Gully, for urban development.
Nearby resident and environmental volunteer Sylvia Netherway said the plans were a double hit to the creek.
�We are concerned about the effect on our already threatened Bush Forever site and the buffer zones, which is a very narrow corridor,� she said. �Increased water run-off and erosion are a big concern when houses are built too close to the creek line.�