Wannon Water also must provide revised plans for a $40m plant expansion to comply with the State Environment Protection Policy for waters, the Environmental Protection Authority says.

The EPA has served the water corporation with a notice to provide more information following fierce community opposition to the proposed expansion during a feedback period.

The expansion would increase the wastewater volume the plant is capable of treating by up to 50 per cent without major technology upgrades to improve treated water quality before it reaches the ocean.

The plant is currently at capacity and the project would prepare the infrastructure for the next 20 years, Wannon Water has said.

Beach campaigners have recorded brown treated solids emitting from the plant's ocean outfall at Thunder Point during events coined a "dirty decant".

Wannon Water is now required to prepare an independent human health risk assessment, including of pathogens that may be present in its wastewater.

The EPA has also requested a health risk assessment, an engineering solution and an explanation of the root cause of the "dirty decant issue".

Wannon Water now must revise its plans for the plant, including showing options to reduce the ocean mixing zone and show that phosphorous levels on the outskirts of the mixing zone would be consistent with state environment policy.

Beach campaigner Colleen Hughson said a community push for upgraded water treatment had drawn people with diverse qualifications including marine science and engineering.

"We want the best outcomes for our marine environment and for future generations, and in order for that to happen, we need a first class sewage treatment plant," she said.

The water corporation has been given until Friday, August 14, to respond to the EPA's requests.

Wannon Water project manager Chris Mahony said responses were being prepared to address all the information requested.

Wannon Water is consulting experts and plans to submit the required documents to the EPA as soon it can.

Have you signed up to The Standard's daily newsletter and breaking news emails? You can register below and make sure you are up to date with everything that's happening in the south-west.