BHP Billiton and Rio Tinto have slammed new WA Nationals leader Brendon Grylls' plan to slug them with a $5 per tonne mining tax as "discriminatory" and "ill-conceived".

Mr Grylls emerged as leader on Tuesday after a two-day marathon meeting, which saw Terry Redman step down from the position.

WA Nationals leader Brendon Grylls says the tax on the mining giants will help fix the state's failing finances. Credit:Erin Jonasson

He proposed the tax on Monday, declaring the government needed "new ideas" to fix the state's failing finances and win the election.

The plan is expected to generate about $3 billion a year.