He said after Wednesday's decision ratepayers, residents and business people would have "even more reason to be concerned".
He said, in his opinion, it was hard to get accurate figures because "information has been withheld" and the project had been handled in a "non-transparent way".
"A seven to four majority is definitely not a good way for [the] council to make such an important decision and speaks volumes of the poor process that has been adopted by [the] council leadership.
On July 28, the council refused to answer 10 questions regarding the proposal's contract and process by the Rotorua Daily Post.
The questions have been re-submitted to the council as a formal Local Government Meetings and Official Information Act (LGOIMA) request.
Hall said he applauded the four councillors who voted against the proposal at Wednesday's council meeting.
"They can call it by another name but it is outsourcing."
On Wednesday council infrastructure manager Stavros Michael told the meeting the contract was not "outsourcing" but "insourcing" and a "partnership".
Yesterday Michael responded to Hall's comments, saying all costs to buy materials and services relating to Rotorua's wastewater would come from local supply chains.
The exception to that was materials that could not be produced here and had to be imported, he said.
In a statement released on Wednesday evening, mayor Steve Chadwick said the council retained "effective control" over the service and management of the contract locked in the certainty of the cost of it over the next 10 years.
"Action was needed to ensure the resilience of our system into the future."
Chadwick said the contract "should be seen" as the council's commitment to protecting the environment, enabling growth in the district and giving certainty that it was taking responsibility and investing in Rotorua's future.
"To not go down this road would have been making the decision to stay with a system that isn't working."
On Wednesday evening, Michael said in a press release the contract was about "proactively identifying emerging risk" and about putting a plan in place that "reduces that risk for an acceptable cost".
He said historical underinvestment in the city's wastewater infrastructure meant there was a high risk the system was "highly likely to experience a critical failure" in the next 10 years.
The impact of such a failure on a critical part of the network could have a "huge" financial and environmental impact.
The council's 30-year infrastructure strategy - implemented in 2015 - saw the council commit to funding the renewal of the wastewater network at about 1.5 per cent each year, an increase on 0.5 per cent previously allocated, he said.
"While this is an improvement, it doesn't have the immediate impact we need to make real change.
"What we have achieved with this contract is a one-network and holistic approach to the management and delivery of our wastewater services.
"We can guarantee the cost of maintaining and improving our wastewater network will increase in the next 10 years for many reasons.
"Without a holistic approach contract, there is no certainty about what this will cost [the] council. With the contract, we achieve a significant degree of cost certainty."
Michael said it was "a different way of doing things" but the development of the proposal had been a "robust, three-year process" with expert input and two rounds of community consultation.
"I am confident that through this contract we are taking another step towards building the resilient and reliable wastewater services our community demands."
The Rotorua Daily Post spoke to some of the 45-odd people who flooded the public gallery at Wednesday's council meeting.
Their comments, as well as Hall's, have been put to the council and Chadwick for right of reply.
Paddi Hodgkiss
"I can't understand why the decision had to be made in such haste. They should have just pressed pause. I had a feeling it was a done deal. The public have been run roughshod."
Lesley Haddon
"I don't know what the implications [of the contract] will be. If things turn sour, it will be like a David and Goliath situation.
"[Reynold] Macpherson was closed down when he tried to go into the financials."
She said she did not think the public understood the details of the contract.
Rose Pemberton
"I totally agree something has to be done but ... are we such a small country we have to go abroad?"
"I haven't heard anything [from the council on the proposal]. No leaflets through the letterbox. I wouldn't even know about it if it wasn't for having a friend who is very knowledgeable [about] the council."