The High Court declaration will have ramifications for all New Zealand councils.
At stake in the David and Goliath battle is the future of the three councils' combined $1.76 billion in ratepayer-funded Three Waters infrastructure servicing 200,000 people.
Mai said the trio was challenging the government over what 'ownership' meant in relation to Three Waters drinking water, wastewater and stormwater assets. The ownership and property rights of these assets was at stake.
A ruling in favour of what the councils say is the traditional view of 'ownership' will likely open the government up to having to pay councils compensation for their Three Waters assets.
Mai said the government's now-mandated Three Waters plans appeared based on 'ownership' that was very different from what was traditionally understood to be the meaning of this word.
She said it was the trio's view the government was expropriating council-owned water assets without conceding it was taking these assets, and without fair compensation being paid to communities for their property.
"Property rights are absolutely fundamental in New Zealand. If you've bought and paid for something, you should have reasonable control over it, and a legitimate expectation that it will not be expropriated without compensation," Mai said.
"As owners on behalf of the community of this critical infrastructure we want to ensure that any future changes respect these basic rights."
Mai said the case could have serious ramifications for all property owners.
"If this goes unchallenged, the simple line to draw is that if the government can single-handedly redefine ownership of Three Waters infrastructure in this manner – then where else could it apply these concepts. Could roads or port companies be next."
She said concerns being highlighted in the High Court case were based on basic and fundamental rights most New Zealanders would take for granted. They would normally not even be up for discussion or a source of contention.
Mai said the government's Three Waters asset shift was undermining basic property rights, setting a risky precedent for New Zealand.
"We don't believe that concepts such as collective ownership with no control of assets and 'shares' that don't deliver any of the usual rights or obligations that go with equity ownership are in line with existing property rights and laws," Mai said.
• Local Democracy Reporting is Public Interest Journalism funded through NZ On Air
