Auckland councillors agreed in principle today to buy 17 new battery powered and electric trains for $207 million, with an initial payment of up to $25m.

It comes as a result of increased patronage on the rail network and the requirement to increase its peak capacity, which will be reached by 2019 when the new trains would come into operation.

The trains have modern batteries which allow them to operate off-grid and recharge when in an electrified section, and will replace existing diesel trains between Papakura and Pukekohe.

Auckland Mayor Phil Goff welcomed the decision at today's finance and performance committee, saying "investing in our public transport system is paying off".

"Aucklanders have taken to public transport with demand increasing by 17 per cent in the last year and we're about to achieve a record 20 million passenger trips a year in Auckland.

"This brings electric trains to Pukekohe over five years earlier than was planned.

"It avoids the rail service reaching capacity and not being able to meet demand by making the extra units available in time, and will continue to encourage commuters to make the mode shift to public transport, easing the increasing congestion on the motorway."

Finance and performance committee deputy chairwoman Desley Simpson says that the decision to purchase the trains would have a significant impact on Auckland's rail network.

"This decision will have a lasting impact on Auckland's transport issues.

"However, I am disappointed that this was not raised during our recently completed Annual Budget with the funding incorporated into that 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."

There are several conditions to the agreement of the purchase, including a commitment by New Zealand Transport Agency to fund at least 50 per cent of the capital and operational expenditure and the timing of the funding aligning to the cash flow requirement of the procurement and operation.

Auckland Transport will also be required to reprioritise its existing capital budget, providing $50 million towards the purchase, and will make an initial payment of at least 5 per cent or $10m, but no more than $25m for the order in September 2017.