The WA Nationals have delivered a critical blow to the West Australian Government's plan to privatise Fremantle Port, refusing to support the proposed $2 billion sale.

Key points: Nationals sink Liberals' plan to privatise Fremantle Port

Nationals sink Liberals' plan to privatise Fremantle Port Say port too important a strategic and commercial asset

Say port too important a strategic and commercial asset Also fear legislation would allow contract details to stay secret

Utah Point and Fremantle Port are part of a raft of public assets the Government has flagged selling in light of spiralling debt and deficits in the forward estimates.

Debt is forecast to reach $39 billion by 2018, and the port is the centrepiece of the privatisation push as the single largest public asset up for sale.

However, Nationals leader Terry Redman said his party could not support the Liberals' proposed legislation to sell such an important strategic and commercial asset.

"My team and I are strongly of the view that we cannot see a pathway, certainly between now and the election, and I suggest after that, that we can have a process... that supports the sale of what is such a significant and strategic asset for the state," he said.

Mr Redman was critical of the Liberals handling of the issue, confirming the three Nationals Cabinet ministers walked out of the Cabinet discussion on the legislation.

The remaining Liberal Cabinet members voted to introduce the legislation to Parliament, forcing the Nationals' hand.

"Our party made the decision at yesterday's party room meeting," Mr Redman said.

It was triggered by the fact that there's intent to introduce legislation into Parliament to enable the sale of the port."

Mr Redman said the Nationals' concerns centre on the enabling legislation, which would allow the Government to reach agreement on a range of issues, including pricing and access arrangements, with any potential buyer.

"If we were to support that, we would have no visibility on any of the contractual arrangements that are so critical for the transparency that our constituency requires and needs for all the users of such a critical piece of infrastructure."

Budget strategy in tatters: Opposition

In Parliament, the Opposition Leader Mark McGowan has lashed the Government over its internal divisions on the port sale issue.

"It is hopeless management of Western Australia. It is hopeless management of government. It is a dysfunction, divided, useless rabble," Mr McGowan said.

Opposition State Development spokesman Bill Johnston criticised Treasurer Mike Nahan, claiming the decision by the Nationals to abandon the sale left the Government's budget strategy in tatters.

"His plan is to sell assets, and yet he can't do it. He can't sell Synergy, can't sell Utah Point, it got referred to a committee yesterday as well, can't sell Freo port," he said.

"No one wants to sell Western Power. His budget strategy is shredded."

Treasurer Mike Nahan returned fire, arguing that in opposing privatisation, the WA Labor had no strategy for funding its big ticket promises like the MetroNet rail network.

"He has no capacity to fund it and he's out there doing what they did at the last election, promising to spend big, fudging the data, and have no plan."

Dr Nahan said private investment was needed to continue to grow the capacity and performance of Fremantle Port, and fund the future second outer harbour at Kwinana.

Government determined to proceed with sale

Fremantle Port was expected to sell for $2 billion.

Premier Colin Barnett said he was "frustrated" by the Nationals' opposition to the sale, but he remained determined to proceed with it.

"We will introduce the legislation into the Lower House and we will only progress it if we are confident we will get it through," he said.

"Otherwise we will go to the election on the privatisation of Fremantle Port and the building of a purpose built, high quality live animal export facility."

Mr Barnett said he believed the Nationals leader had previously supported the sale of the port, but had been placed under pressure by some of his backbenchers.

"I think what's happened is that some National Party backbenchers are objecting to the sale of the port, and I think that's put Terry Redman in a very difficult position," he said.

"Terry was supportive of this initially."

It was a suggestion Mr Redman rejected.

"We're a strong team... and we unapologetically represent regional Western Australia," he said.

Mr Redman also dismissed the suggestion that the Nationals had delivered a critical blow to the government's privatisation and budget repair effort.

"It was always understood right from the outset that the Nationals would treat each issue on its merits," he said.

"We've done that. And we've done it in good faith.

"We've had discussions with the Treasurer in particular about the pathway to how this might happen, in our view that's been ignored."

Further complicating the sale, federal Treasurer Scott Morrison has tightened foreign investment rules, requiring the Foreign Investment Review Board to approve the sale of critical infrastructure belonging to the states and territories.