WA Premier Mark McGowan has confirmed a senior media adviser who quit his position following an incident involving "unwelcome physical contact" at a staff Christmas party previously apologised over a separate incident.

Key points: Stephen Kaless quit over what the Premier called "inappropriate and unacceptable conduct"

Stephen Kaless quit over what the Premier called "inappropriate and unacceptable conduct" The woman who complained about his behaviour is reportedly being supported

The woman who complained about his behaviour is reportedly being supported Mark McGowan says the 2017 incident was "dealt with appropriately"

Stephen Kaless, a senior media adviser to Treasurer Ben Wyatt, resigned on Monday morning after the Department of Premier and Cabinet began investigating the incident.

Mr McGowan said the "unwelcome physical contact" with a woman, who has not been identified, occurred at a staff Christmas function on Friday evening and resulted in Mr Kaless being suspended indefinitely by the WA Government.

He subsequently tendered his resignation yesterday.

Mr Kaless had been Mr Wyatt's key media adviser since Labor's 2017 election victory and also previously worked as the chief of staff of the Channel 7 Perth newsroom.

Mr Kaless was suspended indefinitely before tendering his resignation. ( Facebook: Stephen Kaless )

Mr McGowan said the incident involved "inappropriate and unacceptable conduct".

"There was some inappropriate conduct involving some unwelcome physical contact and that obviously meant that Mr Kaless had to make a decision to resign," he said.

"There will be a further investigation and inquiry by the department.

"We are obviously providing support around the complainant to make sure that her interests are protected."

Police attended the Treasury office in Dumas House and took a statement from the woman late yesterday.

McGowan defends handling of 2017 incident

Asked whether there had been prior complaints about Mr Kaless's behaviour, Mr McGowan referred to a 2017 incident which resulted in an apology.

"There was an incident that was dealt with appropriately by the department back in 2017," he said.

"An apology, as I understand it, was issued in relation to that matter and the matter was therefore dealt with."

Mr Kaless worked for Channel 7's Perth newsroom for four years before joining Mr Wyatt's office. ( Facebook: Stephen Kaless )

Mr Wyatt said he was not aware of any other incidents involving Mr Kaless, apart from the most recent occasion and the 2017 incident.

"These things are tragic for everybody, for the victim, for Mr Kaless, for his family and for my office," he said.

"But ultimately the behaviour meant his position was untenable, and Mr Kaless obviously worked that out pretty quickly and resigned his position."

The nature of the previous incident is not known.

Woman also a member of staff: Wyatt

Mr Wyatt said the woman was a member of his staff.

Treasurer Ben Wyatt says Mr Kaless's behaviour "meant his position was untenable". ( ABC News: James Carmody )

"We will be supporting her in whatever decisions she makes because ultimately out of all the victims, she is clearly the one front of mind," he said.

"I have worked with [Mr Kaless] very closely since he became my press secretary.

"So [I'm] incredibly disappointed, also incredibly saddened about the impact on his family of course, on his children, on the victim, on him."

Liberal MP Tjorn Sibma said the revelations about Mr Kaless's behaviour reflected badly on the McGowan Government.

Stephen Kaless was "suspended indefinitely" before quitting, the Government says. ( LinkedIn: Stephen Kaless )

"I think it's shocking that allegations have been made about Mr Kaless's behaviour that go back a good two years and nothing seems to have happened over that time," he said.

"I think it reflects very poorly on the leadership of both Ben Wyatt [and] Mark McGowan."

The ABC has contacted Mr Kaless for comment.