A new Auckland rail platform which won't be long enough for the three-car electric units due to be used from 2013 can be extended in the future.

The platform at the new Onehunga station, due to be opened in September, is 55m long, at least 15m too short for the new three-car units which will be delivered once Auckland's network is electrified.

Auckland Regional Transport Authority (Arta) is planning to use two-car diesel trains once the station opens.

Arta communications manager Sharon Hunter said the platform length had been set by Auckland City Council during the consent process at the beginning of the year.

"We consulted thoroughly with the community. It was a public and transparent process," she told NZPA.

"In future there is room to extend the platform, though it will need to go through a new consent process and neighbours will need to be consulted."

The current consent requires new rail lines to be at least 14m away from apartments off Princes St.

Ms Hunter said patronage studies suggested there would be about 160 passengers using the station between 7am and 9am, for which two cars would be enough.

The Onehunga line, which has not been used by passenger trains for decades, will open on September 19. The line goes from Onehunga to Britomart, joining the existing southern line at Penrose.

One other new station will be created for the Onehunga line at Te Papapa. Ms Hunter said the platform there would be big enough for the new units.

The platform could in future be extended even more if a line to Auckland Airport is constructed, a line which could go via Onehunga, though such a line is likely to be at least a decade away.

The platform length has angered Auckland Regional Council chairman Mike Lee, who told the Herald the station hadn't been future-proofed, which he called "sheer incompetence".