The CEO of Australia's largest law firm has left after her company took on Attorney-General Christian Porter as a client.

MinterEllison chairman David O'Brien says CEO Annette Kimmitt will leave the firm immediately, just a week after she revealed her concern and disappointment that the company had accepted Mr Porter as a client.

Ms Kimmitt sent an email to all staff last week, hours after Mr Porter publicly denied raping a 16-year-old girl when he was a student in 1988.

“The nature of the matter is clearly causing hurt to some of you and it has certainly triggered hurt for me,” Ms Kimmitt wrote.

“I know that for many it may be a tough day and I want to apologise for the pain you may be experiencing.”

MinterEllison senior partner and defamation expert Peter Bartlett will be acting for Mr Porter.

Ms Kimmett said she found out about the new client through social media, but said the firm’s involvement in the matter should have gone through the firm's “due consultation or approval process”.

“Had it done, so we would have considered the matter through the lens of our Purpose and our Values,” she wrote.

Staff at the law firm are reportedly divided over the issue, with some believing Mr Kimmitt went too far by criticising a senior partner, while young staff in particular say they do not want the firm involved in the Porter matter. 

There is also a perception that the CEO was unfairly pushed from the position, leaving some to question what it means for the role of other senior women.

Ms Kimmitt had worked with consulting giant EY before she pulled off the rare feat of being a non-lawyer hired to lead a top law firm.

MinterEllison has appointed senior Sydney partner Virginia Briggs as its interim head.