The junior said the lightning struck at 4pm on Saturday afternoon, and while there is no visual damage evident at the cold vent where the strike occurred and no one was injured the shut-in was prudent course of action to allow for an independent engineering assessment.

There was a small fire at the vent outlet that was put out by staff and the Gingin fire brigade.

Once the vent has been assessed and government approvals have been granted the plant will be restarted.

The lightning protection system in the plant operated as designed, protecting the plant's electrical systems, Empire said.

"While we were unlucky that the vent was hit by lightning, I am especially pleased that our employees are safe and that the systems designed to respond to a random incident like this worked as intended," Empire CEO Ken Aitken said.

"I'd like to especially thank the local fire brigade for responding so quickly, particularly on a Saturday afternoon.

"The plant will be restarted once all safety, engineering and regulatory issues are satisfactorily cleared, which is expected to be on Tuesday morning."