They said Northland was Aotearoa New Zealand's fastest-growing region with the country's worst roads and highest levels of deprivation.
The Government wants to combine the provision of four councils' Auckland to Cape Reinga drinking, waste and stormwater into a new standalone separate entity – entity A - combining their provision currently separately provided by Whangārei District Council (WDC), Far North District Council (FNDC) and Kaipara District Council (KDC) and Auckland Council through Watercare.
"Two of the three councils (WDC and FNDC) of the North have provisionally opted out of the proposed reforms and the third (KDC) has little confidence in the proposals and despite repeated questions has insufficient information with which to consider either opting in or opting out of the proposed reforms," the mayors' letter said.
"Taken together, the view from the north, of the proposed reforms is not favourable.
"Our concerns are for nga tangata katoa o Te Tai Tokerau who directly elected the three of us to represent them."
Northlanders' wellbeing was of 'paramount importance' to the three mayors, the letter said.
"Considering the current limited confidence in the proposals and the awaited information, the mayors of the North/Local Government New Zealand (LGNZ) zone one (the Northland zone among seven nationally) request you pause the three waters reform programme."
Far North Mayor Carter is an LGNZ board member.
The letters follow each Northland council's individual three waters feedback to the Government.
"Each council has provided separate feedback (on three waters) and this letter does not duplicate that information. Instead it provides a collective view from the North," the Mayors said in their letter.
"We have no sight of the next steps of your programme as this has never been shared with us, therefore we assume you will easily be able to pause this programme in just the same way as you initiated it," they said.
The three mayors make up three-quarters of the new entity A.