Fortescue Metals Group's external and government affairs boss, Deidre Willmott, has resigned.

Fortescue Metals Group's external and government affairs boss, Deidre Willmott, has resigned. Photo: Ross Swanborough

BILLIONAIRE Andrew Forrest believes his crusade against Australia's mining tax is a fight against ''evil''. But after the events of Friday he has even fewer lieutenants to take on that fight.

Mr Forrest's company, Fortescue Metals Group, was rocked on Friday by the resignation of its external and government affairs boss, Deidre Willmott.

The circumstances remain unclear, but Ms Willmott's move continues a period of high turnover within the iron ore exporter.

Among the scores of workers sacked by Fortescue during a sudden debt crisis in September were external relations executives like Julian Tapp and Rod Campbell.

Ms Willmott is well known in Melbourne business circles for her time as a top executive in the organising committee of the 2006 Commonwealth Games.

In Perth, she is better known for Liberal Party connections, having been preselected for the seat of Cottesloe only to stand down when Premier Colin Barnett reversed his retirement plans.

Ms Willmott has joined corporate advisory firm Cannings Purple as executive chairman.

Her departure came on the day that Mr Forrest told ABC radio his High Court challenge against the mining tax was about conquering evil. Sampling from the proverbs of Irish philosopher Edmund Burke, Mr Forrest said evil would triumph when good people do nothing. ''We've decided not to do nothing,'' he said.

Mr Forrest also defended Fortescue's recent unsuccessful attempt to have more than $200 million in royalty payments to the Western Australian government deferred, saying the bid was logical.

Mr Forrest said the deferral was sought to ensure that expansion works at the Kings iron ore project could resume, after a dip in the iron ore price to $US90 a tonne during September halted work on the project.

Mr Forrest said the WA government would enjoy much bigger economic benefits from the Kings project going ahead than from a single royalty payment, and the deferral was designed to ensure the expansion could continue if iron ore prices stayed around $US90 a tonne.

But he said the request became redundant when the iron ore price recovered to about $US120 a tonne today. ''We put that logic to the WA government,'' he said.

''They can see the logic absolutely, but with the iron ore price going from $US90 a tonne to $US120 a tonne they said, 'Guys, you can bring on Kings whenever you like'.''

Fortescue shares fell 6¢ to $3.90.