Western Australia's Treasurer and shadow treasurer have offered to resign if their parties fail to honour key commitments, as they clashed over proposals for a mining tax and an outer harbour at Kwinana.

Mike Nahan and Ben Wyatt clashed in front of a packed crowd of 580 business executives at a pre-election event staged by the Committee for the Economic Development of Australia.

It was the only direct debate between the two opposing treasury spokesmen ahead of the March 11 poll, with Dr Nahan defending the Government's economic record and Mr Wyatt promoting Labor's credentials for restoring the state budget to surplus.

The WA Nationals want to balance the state's books by charging Rio Tinto and BHP Billiton $5 for each tonne for iron ore mined — a plan they say would raise $7.2 billion over four years.

Treasurer Nahan was questioned by the debate moderator on the Liberals' opposition to the tax, and whether it would hold if the Barnett Government was re-elected.

"We will not support or agree to an increase in the user charge like Brendon Grylls suggests, or a change in the royalty system or the royalty rate," he said.

Mr Wyatt cast doubt on the Treasurer's assurance, saying if the Liberals found themselves relying on the Nationals to form a minority government, they would cave in.

"Brendon Grylls has said the Liberal Party will wilt," he said.

"Perhaps the worst secret in politics is that it may not be five bucks, but it will be an amount of mining tax that the Liberal Party will agree to."

Dr Nahan flatly rejected the claim and offered to quit if he was wrong.

"Give you a guarantee, if we do a government and we do get a coalition with the Nationals and that party puts together a Brendon Grylls mining tax, I will resign from the Treasury," he said.

It was not long before Mr Wyatt matched his opponent's resignation pledge as the pair clashed over infrastructure priorities.

It began when Dr Nahan criticised Labor's Metronet project, including the commitment to build a rail line to Ellenbrook.

He said unlike the $1.9 billion Perth Freight Link, the Ellenbrook line had not been either prioritised or approved for federal funding by the national infrastructure body, Infrastructure Australia.

"It is a question of why Labor is now committed to that. There's no business case for it," he said.

Mr Wyatt said the Government had committed to its own major rail project, the Forrestfield-Airport Line, without approval from Infrastructure Australia.

Kwinana outer harbour a 'mirage': Nahan

Ben Wyatt and Labor say an outer harbour is a better transport solution for Perth's south. ( ABC News: Andrew O'Connor )

But Dr Nahan went on to criticise Labor's commitment to axe the Roe 8 highway extension and abandon the Perth Freight Link, in the hope of re-allocating Commonwealth funding to its Metronet plan.

Labor wants to then develop a new outer harbour at Kwinana instead of the freight link.

"You can't do what you're doing. Rip up a contract for a fully assessed project by Infrastructure Australia and put it to a project that has no business case," Dr Nahan said.

He said Labor's plan to build a new outer harbour in Kwinana instead of the freight link was a "mirage".

"What is going to happen if Labor wins? You're going to kill Roe 8, and you're never going to build that outer harbour," he said.

Mr Wyatt said he would stake his job on the proposal.

"If I can borrow your terms, Mike. If we win in 2017 and we don't get that [outer harbour] underway, I will resign," he said.

"We're both resigning," Dr Nahan replied.

But with four weeks until election day, not just yet.