The ABC board says it called in an independent adviser the day before Michelle Guthrie's sacking, to investigate "matters" raised by the former managing director.

Key points: Former MD Michelle Guthrie outlined concerns in a letter to the board on September 21

Former MD Michelle Guthrie outlined concerns in a letter to the board on September 21 The board says it appointed an external adviser to investigate those matters on September 23

The board says it appointed an external adviser to investigate those matters on September 23 On September 24, Ms Guthrie was sacked as managing director

In a statement released this morning, the board said it appointed an "external, independent expert adviser" and that given the investigation is now underway, "it is not appropriate for the board or the ABC to make any further comment pending its completion".

The statement said the investigation would look into matters raised by Ms Guthrie in a letter sent to the board on September 21, just days before she was sacked.

"In that letter, Ms Guthrie responded to several issues that the board raised with her. In addition, the former managing director raised other matters that she requested the board investigate on a confidential basis," the statement said.

The statement said the board "resolved … to appoint an external, independent expert adviser to investigate these matters" on September 23.

One day later, Ms Guthrie was sacked.

Ms Guthrie initially said she was "devastated" at the move, and would consider her legal options. She was fired just halfway through her five-year term.

ABC chairman Justin Milne said the decision had been made in the "long-term interests of our own people and the millions of Australians who engage with ABC content every week".

Mr Milne later stepped down after he was accused of interfering in the broadcaster's editorial independence.

A spokesperson for Mr Milne confirmed the ABC board's statement, telling ABC News the inquiry was discussed at a board meeting on September 23 and that Mr Milne had voted in favour.

"Mr Milne welcomes the inquiry and looks forward to the facts being established," the spokesperson added.

An ABC spokeswoman declined to name the "external, independent expert" adviser.

"As noted, it [the matters raised by and with Michelle Guthrie] is being investigated and managed on a confidential basis," the spokeswoman said.

"I don't have any more information other than what is in the statement."

The decision not to name an external adviser is unusual, given that advisers handpicked for their independence, integrity and experience of such appointments are typically named as part of transparency.

Last week, Communications Minister Mitch Fifield appointed the secretary of the Communications Department, Mike Mrdak, to undertake an inquiry to "establish the facts" about the departures of Ms Guthrie and Mr Milne.