Rail workers have been driven to striking against their will to avoid having money "ripped" from their pay packets by foreign operators, the union says.

Tomorrow will bring the biggest strike to hit Wellington's rail system since 1994, as the union battles with Transdev, which operates the service.

There will be no trains from 2am Thursday to 2am Friday, and no replacement buses for passengers.

"They purchased, on the first of July, 2016, a great little rail operation, which was world class and had been operating very well for a long, long time," Rail and Maritime Transport Union spokesman Wayne Butson said.

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"In the last 12 months they have destroyed the relationship between the union, they've smashed the morale of the workforce, they've turned the workforce to a point where 90-odd per cent of them are prepared to vote for strike action - if that's turning this business around to be a world-class success then actually I don't know how they're in business."

Butson said Transdev were trying to take away long-standing terms and conditions for workers, including double-time pay on Sundays and the ability to decline to work on a statutory holiday.

"Here we are in 2017 with foreign multinationals coming here, wanting to rip money out of our men's pay packets and transfer it to the profits in Paris and Seoul, and that's completely unacceptable.

"It's all about power. It's all about taking away any flexibility and discretion that the employees have and transferring it to the employer."

Other changes Transdev were trying to bring about included requiring workers to get a medical certificate after three days off work sick, rather than four, and making workers pay for it instead of the company.

Butson said Transdev also wanted to drop staff numbers to "unsafe" levels.

Since they took over operation of the service, trains had consistently been staffed at the minimum level rather than the desired level, he said.

The union was fighting for "real wage growth", disclosure of any plans for change in the company, and a living wage for certain workers in "low-paid designations".

"They're a glorified labour company ... If they need to wrench terms and conditions off our members to increase the profits on their balance sheets then it isn't going to happen with this union being involved."

Butson said they had agreed in principle to two of Transdev's claims, but would not allow core terms and conditions to be taken away.

"We're really sorry that [commuters] are being inconvenienced, but unfortunately the only way that workers in New Zealand can fight back is by striking ... We've been driven into taking action against our will simply because we want the employer to see sense.

"We want someone, whether it's Greater Wellington Regional Council or anyone, to step in and just get these rogue employers to negotiate in a meaningful way."

But Transdev manager of people and culture David Gould said they were trying to push a good deal for all employees, which included a two per cent pay rise for all staff.

"The fundamental point here is that we have a great offer on the table that will benefit all staff. This union will not accept a single iota of change and that's standing in the way of a great rail passenger service for Wellington."

Transdev wanted to reduce staff numbers on off-peak services and increase them on peak ones. They also wanted to introduce innovations such as integrated ticketing.

"The reality is that this union are hell bent on strike action and they have been since before they initiated bargaining.

"Their position is they will bargain with us only if we remove all our claims. That's not bargaining, that's a stick up."

People should allow extra time tomorrow for travel, and expect heavier traffic on the roads.

Students commuting to get to exams should contact their school or faculty office for advice if they believed they would not be able to make it in.