Acting Prime Minister Mathias Cormann has described the sexual harassment complaint made against Deputy Prime Minister Barnaby Joyce as "very serious" and a Nationals MP has escalated the internal pressure on his leader, declaring "all bets are off".

While Mr Joyce has branded the latest allegations "spurious and defamatory," the Nationals executive said the formal complaint from a Western Australian woman would be taken seriously and given due process.

"Some very serious allegations have been made. All allegations of sexual harassment are very serious. It's important that we ... establish the facts," Senator Cormann said.

"Barnaby Joyce has denied the allegation, but it's being investigated and subject to the outcomes of that investigation, obviously, the appropriate steps would follow."