It comes as an "imbalance" caused the plant to periodically discharge organic suspended solids into the ocean mixing zone at Thunder Point this week.

Wannon Water managing director Andrew Jeffers said the plant was "approaching capacity" and had imbalances in the different types of loads from domestic and major customers.

"We are aware of this issue and are making operational changes at the sewage treatment plant to help minimise the impact on the environment," Mr Jeffers said.

Beach campaigner Colleen Hughson and the Clean Oceans Foundation are pushing for better water treatment at the facility, which could include tertiary class A treatment that removes inorganic compounds from sewage water.

Wannon Water is nearing an upgrade, currently under EPA review, that would double the plant's capacity at a cost of $40 million, using similar technology already at the plant with the addition of a new inlet screening facility.

Ms Hughson said she was concerned brown murky water had been discharging from the plant's ocean outfall occasionally since March.

"I think they need to be looking at solutions to the problems they've been having," she said.

Mr Jeffers said the $40 million upgrade was "the first step" in a long-term plan.

"(It) will enable future improvements at the site, such as enhanced nutrient removal, tertiary treatment and/or the production of recycled water for commercial reuse when the timing is right," he said.

But the water authority says an upgrade for ocean discharge tertiary treated water would cost a further $30 to $40 million, an investment of around $1700 to $2300 per customer bill.

Wannon Water would need broad support from customers before considering the option and then making a submission to the Essential Services Commission in 2023.

Some costs could be recovered if Wannon Water was able to sell recycled water to industry but it has had no expressions of interest and found "currently no driver to produce recycled water".

Ms Hughson said she wanted governments or industry to cover the upgrade cost, not households.

"It should be a federal or state government cost, or industry should be putting in," she said.

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