Ms Carles told reporters last week she did not want to make it a public issue but felt the pressure from Greens leader Giz Watson.

"The last thing I wanted to do was go public because of the impact on the families involved," Ms Carles said.

"However, pressure was mounting last week, my Greens colleagues knew, they felt they had been compromised and that they couldn't withhold this information."

Ms Carles, who is married with three daughters, confessed to the "brief" affair Sunday, April 25.

Mr Buswell, 44, is also married, with two sons.

"It is with deep regret that I acknowledge having an affair with the Treasurer, Troy Buswell," Ms Carles said on Sunday.

"It lasted several months and it is over now.

"I could attempt to portray myself as the vulnerable one who was taken advantage of. However, this is simply not true.

"We made a mutual, albeit stupid, decision as two consenting adults.

"To my constituents in Fremantle, I apologise."