The day before his resignations, Milne fired Guthrie, ABC's first female managing director, saying that the board of directors had determined it was in the network's best interests to let her go.
The nearly century-old ABC is incredibly popular Down Under, with polls showing that it is not just the most trusted news organisation in the country, but also seen as a national treasure.
ABC coverage is legislatively obliged to be independent and unbiased, but - for reasons either real of imagined - it is rarely perceived this way, particularly when top investigative programs expose stories which are inconvenient and embarrassing to those in public life.
"There was absolutely no interference in the independence of the ABC by the government".
Mr Milne said there was no political interference from the government in his decisions.
The ABC's board selects the managing director, while the governor general of Australia appoints the board and chairman at the recommendation of the government.
Milne had allegedly told Guthrie to sack two senior journalists whose reports had provoked complaints from the government this year.More news: Plane overshoots runway in Micronesia and lands in ocean
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Mr Milne's spokesman did not respond to a requests comment on the Alberici email on Thursday.
The ABC board met on Thursday morning without Mr Milne, after which the board asked him to step aside.
"Their responsibilities as a director require them to devote extra time and attention should their company be facing problems - we have even seen some cases where they have stepped into the role of acting CEO", Ms Peres da Costa said.
Hours before Milne's resignation, the communications minister, Mitch Fifield, indicated that Milne no longer had the support of the government.
"Nobody from the government has ever rung me and told me what to do in relation to the ABC", he said. "But center-left governments have also complained of unbalanced reporting in the past".
Media mogul Rupert Murdoch (L) and then Prime Minister Malcolm Turnbull (C) in a 2016 file photo.
MALCOLM Turnbull reportedly hated a star ABC journalist so much the chairman called for him to be fired, a claim the former PM denies.
The journalists' union, the Media, Entertainment and Arts Alliance, has called for Mr Milne to quit.
Milne strongly denied on Thursday that he had attempted to interfere in the ABC's editorial independence.