A planned 10 per cent levy on taxi and ride-share trips in WA appears all but doomed, with the Liberal Party and key crossbenchers vowing to oppose the controversial so-called "Uber tax".

With the Liberals and One Nation both confirming they will oppose the proposed levy, the Government's proposed taxi reforms have been left in tatters — with Labor maintaining only slim hope of implementing its plans.

Under the Government's plans, revenue from a 10 per cent levy would be used to fund a buy-back of taxi plates which have plummeted in value since the emergence of Uber and other ride-sharing competitors.

But Liberal leader Mike Nahan said his party could not support a levy.

"They never indicated they would put a tax on to fund it and they promised West Australians [not to introduce] new taxes," he said.

"Before the election they promised the world, the sun and the moon without any funding, and now they're stuck."

One Nation leader Colin Tincknell also confirmed his party would not back the plan, joining Liberal Democrats MP Aaron Stonehouse in opposing the levy.

Revenue from the levy would have been used to fund a buy-back of taxi plates. ( ABC News: Andrew O'Connor )

That left the Government with slim hopes at best of getting the measure through the fractured Upper House, with the Greens also previously having voiced their opposition to an Uber levy and the Nationals also previously critical of parts of the reform package.

The Nationals are awaiting a briefing from Transport Minister Rita Saffioti before making a final decision, but even their support alone would not be enough for the levy plan to pass.

The Greens also said they had yet to reach a decision on how they would vote, saying they would be discussing with stakeholders before announcing a final position.

No broken promise: Minister

Ms Saffioti said if the levy was blocked, the whole scheme would be dead and taxi plate owners would not get any further compensation.

"If the Liberal Party wants to walk away from small business owners … then it is up to them," she said.

"We are delivering a package here and it is a whole package."

Ms Saffioti rejected suggestions the levy was a broken promise, saying Labor had always made clear that it would fund a voluntary taxi plate buy-back via the industry.

The Government also pointed to other previous statements from Liberal MPs such as Tony Krsticevic and John McGrath — indicating their support for a levy — to accuse the party of hypocrisy in opposing the plans.

The Government's plan would see the 10 per cent levy imposed on operators rather than passengers, but Uber and other services have indicated that would be passed on to passengers.

The buy-back scheme, if implemented, would guarantee the owner of a conventional metropolitan taxi plate at least $100,000 in compensation.