Tomorrow's strike will be the biggest to hit Wellington's rail system since 1994.

The capital will have no trains at all tomorrow, as members of the Rail and Maritime Transport Union hold a 24-hour strike, from 2am Thursday to 2am Friday.

More than 30,000 passenger journeys are made by train on a regular weekday in Wellington, but now those commuters are being told they should consider staying at home.

Union spokesman Wayne Butson said it was the first industrial action since 1994 to affect Wellington's rail system for longer than two hours.

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He said the union didn't make the decision lightly.

"We've been trying to negotiate with Transdev and Hyundai since May this year, and they've stalled every single step of the way.

"We all regret the disruption this will cause for Wellington commuters, and we encourage them to ask the regional council, and their mayors, some hard questions about why big international companies are being allowed to run down good Wellington jobs."

Butson said the two multinational companies were demanding the removal of long-standing terms and conditions in the collective agreement.

He said strike action was the only option they had left "to get the boss to sit down and be reasonable".

Metlink's manager of rail operations Angus Gabara said they couldn't put on that many replacement buses for such a high-use service.

He said commuters would need to make other travel arrangements, and if possible, not travel at all.

"Commuters, people with medical appointments, and students sitting exams who have to travel, should allow extra travel time due to the disruption this strike will cause.

"Bus services are operating as usual but we expect them to be under significant pressure with increased passengers.

"Motorists should also expect delays because of the extra cars that are expected to be on the roads."

Gabara said they were still looking at options to help travellers who had no alternatives ways of getting around.

He said any last-minute changes would be posted on metlink.org.nz.