A $2-million-plus mining lobby campaign against the WA Nationals' proposed mining tax appears to have driven voters away from the party in key mining areas.

Polling trends in Western Australia's mining and pastoral regions in the north and east of the state show a drop in the Nationals vote in some seats of more than 10 per cent.

The trends contrast sharply with those experienced by Nationals MPs in the agricultural regions in the state's south west, where the Nationals vote largely held firm.

"The National Party vote held up or even went up in parts of urban south-west Western Australia," ABC Election Analyst Antony Green said.

"The vote held up across the agricultural districts, at most down about 4 per cent. But in the mining and pastoral districts, in Pilbara and Kalgoorlie, we're talking about the National primary vote being down 10 per cent or more," he said.

The WA Nationals campaigned heavily on their policy to increase an existing iron ore production rental fee of 25 cents per tonne paid by miners Rio Tinto and BHP Billiton, to $5 a tonne.

WA Nationals Leader Brendon Grylls argued the move would create a new revenue source of $7.2 billion over four years, and was the only way to repair the debt and deficit-laden West Australian budget.

Brendon Grylls making the case for the tax at a Nationals' business breakfast in Perth. ( ABC News: Andrew O'Connor )

Mining industry set sights on Grylls

In December, it became clear the mining industry's strategy was to eliminate the tax by targeting the architect of the idea.

As he did the hard sell on his policy, Mr Grylls faced an advertising avalanche from the mining lobby, with the WA Chamber of Minerals and Energy bankrolling a multi-million dollar campaign across radio, television, print and social media.

The CME also relentlessly prosecuted its argument against the tax in media interviews and a radio debate leading up to the March 11 poll.

And according to ABC Election Analyst Antony Green, it worked

Mr Green said the poll results, including in Mr Grylls own seat of Pilbara, show the policy and the mining campaign against it, turned voters away from the Nationals in mining areas.

"I think what you're seeing is the sheer amount of money that was spent on the election has damaged the Nationals' prospects," he said.

Brendon Grylls is trailing his Labor opponent Kevin Michel in Pilbara, and looks unlikely to close the gap.

"I can't see anything that will save him," Mr Green said.

Brendon Grylls attempts to sell his mining tax proposal on ABC Radio. ( ABC News: Andrew O'Connor )

In a statement issued after Labor's landslide win, the WA Chamber of Minerals and Energy welcomed the Labor Government and the end of the National's mining tax.

"As a former state development minister and minister for environment, Mark McGowan has a good understanding of the issues facing the resources sector," CME chief executive Reg Howard-Smith said in a statement.

"We are pleased the Labor Government will not implement the job-destroying iron ore mining tax as proposed by the WA Nationals. Based on the election results there is no mandate for this tax to be implemented and CME is pleased Labor stand firmly against this bad policy."

Nationals may not 'walk away' from policy

But the WA Nationals have yet to abandon the policy, even in the face of what appears to be a clear electoral backlash in the mining areas of the state.

Nationals MP Colin Holt, a former party president and Barnett government minister, said the result in the state's north was due largely to the mining lobby's campaign against the tax, but denied it represented a rejection of the policy.

"It is a reflection of the massive campaign that occurred in those areas," Mr Holt said.

"It doesn't mean that we walk away from a policy like that at this point in time."

Mr Grylls has been unavailable for comment.