The upshot for ratepayers is the council will be paying $100,000 a year less than expected in debt servicing with $1.5m less debt on the books.
That will mean lower than expected pan tax rates and trade waste fees which reduces the average rates rise for this year by 0.1 per cent to 4.5 per cent.
Construction of the new plant was completed in March and is now in a biological commissioning stage.
"We're now halfway through the commissioning process and the plant is already delivering well beyond our expectations," council chief executive Kym Fell said.
The cost coming in under budget was great news, he said.
"It's a result that can be attributed to 18 months of exceptional management from the council's infrastructure team, along with technical expertise provided by global firm Cardno and construction company Downer Group."
The saga over the previous plant - which was began operating in 2007 but never met its resource consent before ultimately failing and being shut down in 2012 - was deemed to have cost ratepayers $27m.
In 2016 a $54,000 independent report by Robert Domm found cost saving was a driver behind the design of the failed plant and that council was misinformed ahead of critical decisions.
"The evidence is very strong that significant flaws at critical early stages of council's decision-making process in 2003-04 allowed an untried and untested plant to be constructed, contrary to the historically more risk-averse and sensible approach of the wastewater treatment industry. Significant risks were downplayed," the report said.
Meanwhile, Fell said the new plant was already exceeding requirements.
"The water we are discharging into the sea at the moment has been through the treatment process, including UV disinfection, and it looks clear and clean as it leaves the plant.
"I am delighted that we are delivering a fully compliant wastewater treatment scheme ahead of time and well under budget that will meet the needs of our community now and into the future."
The new plant is now operating 24/7 and council's infrastructure general manager Mark Hughes said he and staff were "fully committed to getting this right for the people of Whanganui".
"We're very proud to be working at such a successful state-of-the-art facility."
The council received an out-of-court settlement from MWH, the designers of the failed wastewater treatment plant, but the figure remains confidential.
The Chronicle challenged the confidentiality but the Ombudsman ruled council was entitled to refuse to disclose the settlement amount but recommended it should disclose a "certain level of information".
Council then said it received "an amount in excess of [both] the total costs associated with the litigation, including legal costs and expert fees, and the professional fees paid to MWH in connection with the wastewater plant".
