While the ABC faces four separate inquiries, three of its most senior executive roles will be vacant, with head of regional and local Michael Mason announcing his resignation on Wednesday.

Mason is leaving after 34 years at the broadcaster, but he told staff his decision was made in early August, before the tension between managing director Michelle Guthrie and chair Justin Milne became public, Guthrie was sacked and Milne resigned.

Mason will leave the broadcaster in December, and former Sydney Morning Herald editor Judith Whelan, who’s been at the ABC for two years, will act in his role from November.

The announcement comes as part of the biggest shake-up of executives in the broadcaster’s history, just weeks after Guthrie’s sacking amid accusations of political interference involving Milne wanting journalists fired for upsetting the government.

Current acting managing director, David Anderson — who applied for the role when it was given to Guthrie — has been actively lobbying for the top job, telling ABC Melbourne radio host Jon Faine on air on Wednesday that he wanted the job permanently.

Anderson told staff in an email on Wednesday that Mason’s departure would be a “significant loss”. “Without a doubt he has been one of the most influential radio executives in Australia and his decision to leave is a significant loss to the ABC,” he said. “He has steered local radio and the national radio networks through some turbulent periods to expertly position us for the future.”

Mason was one of the execs who kept his job as head of a department under Guthrie’s restructure that was meant to remove the divide between TV, radio, online, and news.

His announcement has been left with wide public praise from current and former staff — a stark contrast to Guthrie’s sacking, where the most high-profile criticism came from Four Corners executive producer Sally Neighbour, tweeting that it was an “excellent decision”.

Mason was not universally loved internally, though. As director of the former radio division, he oversaw cuts that were seen in some quarters as a “dumbing down” of Radio National.

Last year’s decision to cut radio current affairs programming from two and half hours a day to 90 minutes — halving The World Today and PM — was under Mason’s watch as director of radio. He was given that position at the end of 2014, at the same time Tony Abbott’s government, with Malcolm Turnbull as communications minister, announced drastic cuts to the broadcaster’s budget. In the years following, specialist programming at Radio National continued to be cut.

Mason told staff that his departure was “in no way linked” to Guthrie and Milne’s departures, and that he’d made his decision in August.

Replacements for all three senior positions will walk into the middle of four inquiries facing the public broadcaster, including the Senate’s vote to establish a committee looking at the ABC’s independence from political interference.