The dilapidated building had been increasingly difficult to put up with, Mai said.
"We know that throughout the country there's been under-investment in hospitals and schools and there's a big job to catch up to bring us all to the same level but it does send a message that, 'oh Northland, you can wait, we'll get to you eventually'."
Mai said original cost estimates would be out of date now and the build would take years to finish.
"It won't be fast, this is a major investment and a major project. It will probably be the biggest investment that our region has seen, ever. So it's really important that we get it right."
The district health board has told the Government since 2017 that overall demand for the emergency department, intensive care, inpatient beds, and theatre services has surpassed capacity - and Northland's medical and surgical needs have been "unprecedented".
Last year RNZ revealed raw sewage was leaking down the inside of walls of the hospital's medical wing.
The piping is taking two years to fix, at a cost of $2.8m, because it is surrounded by asbestos.
Last year, Health Minister Andrew Little said the hospital was one of "a number" of priority redevelopments needed.
In a statement on Friday, his office said the cost of the Northland DHB's redevelopment business case, Project Pihi Kaha, was originally put at $572m.
The minister confirmed the Government had ringfenced that amount, "subject to the business case".
He expected this latest version to be submitted by mid-2022.
Whangārei MP Emily Henderson told RNZ the hospital was in "shocking" condition.
"Our hospital is crap, but our staff are amazing and they have been working in really shitty situations, shitty conditions, literally as we all know, for some years. And I am desperate to give them the facility they deserve."
She said Little was well aware of the disrepair - she took him on a tour last year to see it, with the chief executive.
"We took him through the bits where there was water dripping through the roof when it rains, we showed him the cracks in the walls that are covered up with bits of plastic. We showed him the operating theatres where windows fall out."
Northland DHB chief executive Dr Nick Chamberlain was aware money had been set aside for the hospital, and he believed it would be distributed across this year's budget and next year's.
"We're making good progress with the business case and have had an independent review of it, which is a standard process. [We've] worked with our clinicians on the detail of that and are planning to submit the business case to the April Capital Investment Committee meeting."
He had previously anticipated funding would be announced in last year's budget.
