Auckland Council has tagged on to buy $207 million worth of electric and battery-powered trains.
The council's finance and performance on Wednesday agreed in principle to buy the 17 trains but it will have to find $25m for an initial payment by September.
The council will also now have to find $50m from its capital budget and get a commitment from the New Zealand Transport Agency for 50 percent of the capital and operational expenditure.
The decision came with a rebuke from committee deputy chairwoman Desley Simpson, who said it was disappointing the funding was not incorporated into the annual budgeting process.
"I appreciate the apology and commitment from Auckland Transport that we will be fully appraised of similar scenarios in the future ahead of time," she said.
It means electric trains will be running between the city and Pukekohe five years earlier than planned, says Mayor Phil Goff.
Mr Goff has said the new units will have major benefits for commuters living south of Papakura in the high growth areas of Drury, Paerata, Pukekohe and potentially Pokeno.
They can operate on lines not yet electrified and would allow the council to eliminate ageing and less reliable diesel trains.
Demand has increased by 17 percent over the past year and Auckland is on course to achieve a record 20 million passenger trips a year within months.