Opposition parties would return water infrastructure to councils, they say, and they doubt the proposed working groups will address concerns about governance, ownership and accountability.
Local Government Minister Nanaia Mahuta confirmed councils around New Zealand would be required to hand over their water assets under the government's three waters reforms.
The reforms would go ahead largely as planned, although working groups would be set up to inform and guide the legislation, with a particular focus on oversight and accountability, rural supplies and resource management.
National's leader Judith Collins committed to repealing the reforms and returning those assets to the councils if National was returned to government.
"Hopefully in 2023 ... we will be returning the confiscated assets," she said. "Ultimately we don't believe that the answer provided by the government, which is all around centralisation, is actually the right answer."
She said after millions of dollars spent - about $4m on television advertising and $710 on consulting with councils - the approach was extraordinary.
"It would be more important, and more sensible we think to target the money, the funding and also the assistance to those who actually need it, not everybody else."
The party's local government spokesperson Chris Luxon said it was an asset grab focused on centralisation, and he by his count 60 of the 67 councils were opposed.
"Frankly, it's taking power and control away from communities."
He was doubtful too about the working group having much influence over the governance concerns mayors had raised.
"[Mahuta] was quite adamant in her press conference that councils will have no direct influence or control over these assets and so therefore, frankly, what's the point of the working group that's already predetermined and preordained?
"The mayors and councils - who are reasonable good people; who understand their communities well; understand these assets well; have managed them well by and large across the country - they've been giving that feedback. And the minister frankly, said, 'Nah, I'm not listening, I haven't changed my proposal from day one and I'm steamrolling it through'."
He said a full select committee process would take four to six months, but by the time it was implemented the local body elections would be looming.
"Politically, you know, the communities are going to be up in arms, the councils and mayors will be up in arms about it and I just think it's very, very difficult and I just don't understand why the government genuinely doesn't take a step back, have a cup of tea.
"Rethink the whole thing, rethink the engagement with local government, and actually come forward with solutions that both parties are actually invested in and can support.
"That hasn't been the process here, it's been a sham process of goodwill but fundamentally 'my way or the highway', and that's what's going to happen. It's just going to get jammed through under majority rule."
ACT leader David Seymour said the party would be campaigning to return "stolen property" if the government went ahead with taking the assets.
"Nanaia Mahuta seems impervious to logic and criticism. If the councils don't think her scheme makes their ratepayers better off, maybe they're trying to tell her something.
"These assets owned by councils are going to be taken over by new entities that councils have next to no accountability over. Ratepayers who could hold their local council accountable are going to find a distant and much more complex bureaucracy is going to be highly unresponsive."
He said if economies of scale were needed, councils could enter into those voluntarily as had been seen in Auckland and Wellington.
"They did it because it made sense to them. Councils actually have local knowledge and they should have the ability to decide when it makes sense for them to cooperate and work together."
"We believe if it's a problem for councils to be able to operate this infrastructure it will be a problem for these new entities. The solutions are voluntary cooperation and long-term cooperation between central and local government on infrastructure as well as GST sharing."
Green Party spokesperson Eugenie Sage said everyone agreed there was a case for reform but the government should not be imposing it on councils.
"There needs to be a genuine conversation, genuine engagement about the concerns that councils around the motu have raised, and change particularly in relation to representation, governance and accountability."
She was also sceptical about the working groups' ability to influence the model.
"Councils have made a number of suggestions, there has been some engagement, but introducing the legislation before Christmas means that there's only seven weeks to have those working parties operate. That doesn't allow time for genuine engagement.
"The Green Party thinks that the council-controlled organisation model has a lot to recommend it, and taking elements of that and putting it into the new entities."