Federal environmental approval for WA's second desalination plant is expected next week, paving the way for work to start on the $1 billion project next month.

This is the final hurdle for the plant after WA environmental approval was granted in February.

Water Corporation spokesman Phil Kneebone said the approval was imminent for the plant, which is needed by 2011 to meet increasing demand for drinking water and ease pressure on the stressed Gnangara Mound.

It will initially produce up to 50GL a year but could be expanded to produce 100GL.

State Environment Minister Donna Faragher imposed strict conditions to limit the plant's impact on flora and fauna, wetlands and the marine environment when announcing conditional approval this year.

Reports for the Federal environmental process said the plant will disturb vulnerable and endangered species of possums and cockatoos, but was unlikely to have a significant environmental effect.

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