March Madness in Auckland could be extra mad this year, with the threat of strike action hovering over Auckland's rail network.

Rail and Maritime Transport Union members will start voting on strike action today, and the decision is expected to be announced in three weeks.

Union members have been unable to agree on a pay increase with Transdev, the French company contracted by Auckland Transport to operate metro-rail.

Transdev managing director Michel Ladrak said industrial action is always a worry.

"It is always a big worry to the company because we're here to offer the best possible services to our customers, so as soon as people take industrial action, it's never a nice thing."

Rail and Maritime Transport Union organiser Rudd Hughes said rail workers are livid they can't reach a pay agreement with Transdev.

The union is seeking a 3 per cent pay increase over the next year, with a 1.5 per cent increase in the following six months.

"We think that's a pretty reasonable claim and we can't understand why the company is being so difficult."

He said if union members vote, workers won't be doing overtime and they haven't ruled out walking off the job for whole days either.

However, Ladrak said he thinks they've offered workers a fair deal.

"I think we've put a very reasonable offer on the table, around 2.3 per cent, including extra sick days. That's very much in line with the rest of the industry around New Zealand."