Auckland might have to stump up hundreds of million of dollars for a new fleet of commuter trains, with rapid growth in the numbers riding the city's rails.
Auckland Transport said the 57 electric trains introduced over the past two and a half years could be filled to capacity by 2019, and some big decisions would be needed next year.
The $500 million investment in the Spanish-built electric trains has been a huge success, and talk of buying more has come up sooner than previously thought.
Patronage was up 15 percent on a year earlier, and the rising trend was stronger and lasting longer than originally forecast, with annual patronage approaching 20 million trips.
The council-owned transport agency was considering whether any new trains would be hybrids with batteries, [http://www.radionz.co.nz/news/regional/305860/batteries-on-track-for-pukekohe-trains which would be able to travel beyond the electricity supply to Pukekohe and Kumeu.
Auckland Transport chief executive David Warburton said the number of new trains that would be needed was not yet clear.
"It depends whether money is put into electrification to Pukekohe for example, it depends on the frequency of services, it could be anything from 15 to 20-plus trains," he said.
Further heavy investment in trains could be a financial challenge for the Auckland Council. Previous mayor Len Brown stripped $330m out of long-term budgets two years ago, arguing the need for trains was farther into the future.
About $250m might now have to be found in the next three years, with a two-year lag between placing an order and taking delivery.
Dr Warburton said the agency would need to be talking to the council next year about how the cost would be met.