Western Australian Treasurer Troy Buswell resigned on Tuesday after revelations that he used public money and resources during his four-month illicit affair with Greens MP Adele Carles was made public and snowballed into calls for him to quit.

Speaking to reporters at Governor Stirling Tower, Western Australia Prime Minister Colin Barnet said he had accepted the resignation of Buswell.

Revelations that Buswell paid for motel rooms in Albany and used the ministerial cars during his trysts with Carles put pressure on Barnett's administration. Calls for Buswell to resign reverberated across the state.

Buswell's resignation came as a complete reversal a day after the ex-Treasurer, accompanied by his wife, told a press conference that he would not relinquish his post and stay with his Cabinet post.

But that all changed when Barnett was told that Buswell misused public funds to finance his encounters with Carles. Barnett said he learned of that fact when he spent time with Buswell at his Toodyay property on Monday.

The premier originally supported Buswell and said he wanted the Treasurer to remain with his Cabinet. He described Buswell as his friend.

"While I consider the relationship between Troy and Ms Carles a personal matter, Troy's actions in relation to the use of his Government entitlements in this matter is unacceptable. As a result, this morning I informed the Treasurer that I believed his position as Treasurer and Minister was now untenable. Troy offered me his resignation, which I accepted," Barnett told the media.

Barnett said Buswell accepted the consequences of his actions, but he lost the job he loved despite being an outstanding Treasurer.

Barnett will take over the Treasurer portfolio just three weeks before the State Budget is handed down and seek to appoint a replacement after that.

He said he learned of the affair last week but he added it was not up to him to judge Buswell and Carles for having an affair. He said it was a very poor judgement for public officials to have an affair.

He also clarified that the affair was not the reason he asked for Buswell's resignation. What was not acceptable was the use of public money, he said.