At 9am yesterday a valve was opened and all Whanganui's effluent was let into the city's new $39 million wastewater treatment plant for the first time.

It's a major milestone for Whanganui District Council and a huge turning point for the local marine environment, CEO Kym Fell said.

"We are no longer sending raw sewage out to sea and that is worth celebrating."

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He was pleased with progress on the project, saying it has tracked according to plan, stayed within budget and is ahead of time.

Infrastructure general manager Mark Hughes remembered sitting before environment commissioners discussing a three-year consent to discharge to sea. He said it was satisfying to end that earlier than expected.

"Everything is going well and we can observe the process through our Supervisory Control and Data Acquisition (SCADA) programme, which enables us to see each stage at a glance."

Last weekend about 1000 people went to an open day at the plant, and feedback was positive.

"It was great to see so much interest and enthusiasm from the community," Fell said.

For the next two months the plant will be in biological conditioning mode. It will be fully operational by July.