Mining giant BHP Billiton has been accused of "bullying its staff" by displaying anti-mining tax advertisements on employee computers and staff rooms.

The ABC understands the ads, which ask staff to consider what would happen if WA mining was made uncompetitive, have been shown on computer screensavers at Port Hedland worksites since November.

WA Nationals leader Brendon Grylls, who proposed the controversial increase in tax on iron ore, said he was aware of the messages being relayed to staff on worksites.

"I'm just aware that they've overridden the computer system to do their anti-Grylls message on the computers of their employees in Hedland," he said.

"It's disappointing that a company as big as BHP would bully their staff into a political campaign but that's their choice. I won't do that, I'll just tell people the facts.

"I understand it's mentioned every morning before work at the musters."

Mr Grylls wants to increase production taxes for BHP Billiton and Rio Tinto from 25 cents to $5 a tonne, to raise an additional $7.2 billion over four years.

In a statement, a BHP Billiton spokesperson defended the anti-mining tax screensavers.

"BHP Billiton has an obligation to inform our workforce of the potential impacts of the proposed iron ore tax and present them with the facts on it," the statement said.

"The tax will have negative flow-on consequences for jobs and investment in the Pilbara and see the majority of any proceeds flow to the eastern states."

BHP Billiton executive Mike Henry also launched a stinging attack on Mr Grylls' proposed mining tax last year, warning it was "punitive" and would cause job losses.

The WA Government today requested Governor Kerry Sanderson issue the writs for the March 11 election, officially kicking off the formal election campaign.