Not to be outdone, US ride-share service Uber is promising heavy discounts to first-time users of the UberX transport app.

Victoria's taxi regulator has granted a one-day all day licence to the state's 533 peak service taxi operators, who are normally permitted to work between the hours of 3pm and 7am only.

Tomorrow the distinctive green-roofed cabs will be free to operate from 8.30am, just as train services begin to thin out in preparation for the Rail, Tram and Bus Union's four-hour stoppage.

The strike is expected to impact Metro train services for almost eight hours.

It is not known how many of the 533 operators will take advantage of the Taxi Services Commission's exemption, but a spokeswoman for the industry said she hoped many would grasp the opportunity.

Melbourne has more than 4800 taxi licence holders and more than 15,000 active drivers.

"There'll be more demand and hopefully there will be more taxis on the road," Georgia Nicholls from the Victorian Taxi Association said. "Drivers should see that there is an opportunity to service the many people who will be left stranded."

Metro estimates 300,000 passenger journeys will be disrupted during the strike.

Demand for taxis soared during last week's similar four-hour tram strike, Ms Nicholls said.

She also took a shot at new competitor UberX and its practice of charging surge prices at times of very high demand.

"Taxi customers can be certain of what they'll be charged, regardless of demand in those busy times," she said.

A spokeswoman for San Francisco-based Uber, which generally undercuts Melbourne taxi prices with UberX, said demand for the service doubled when the tram system was shut down last week and the company expected a similar situation on Friday.

"When we became aware of last week's action, we let all our driver partners know that Melbourne would need help moving around," she said. "In response we saw twice as many as usual come online, who carried twice as many passengers as the same period the week before."

The company will seek to make the most of the Metro stoppage to expose many more people to its service. First-time users who enter a promotional code when downloading the app will get a $30 discount, or a free ride up to that amount.

Uber said it would bear the cost. UberX relies on drivers who use their own cars to transport passengers.

UberX vehicles are not accredited by the Taxi Services Commission.

For this reason, Taxi Services Commissioner Graeme Samuel has said the service is "illegal".