Mr Howard, who once called Mr Mugabe a "grubby dictator", said it was hard to imagine how Zimbabwe could get worse if the military coup underway in the poor and brutalised nation forces him from power after 37 years.

He called on South Africa now to use its clout as the dominant power among the southern nations of the continent to push for democratic change, though he said he was not hopeful this would happen.

Mr Mugabe, 93, was on Friday under house arrest in the custody of the military amid what appears to be a power struggle between his wife Grace Mugabe and recently sacked vice-president Emmerson Mnangagwa, a former long-time Mugabe ally.

Mr Howard, who clashed with Mr Mugabe in the early 2000s over his disastrous economic policies and brutal leadership, said Mrs Mugabe might be "almost as bad" and Mr Mnangagwa appeared "almost equally odious" but added that he was "delighted" by news of the coup and found it "hard to believe anything could be worse or as bad as Mugabe".

"Plainly it has to be better news in the long run for Zimbabwe if Mugabe is removed," Mr Howard told Fairfax Media.

"He used dictatorial methods. He was just as odious as the white regime he so despised and criticised, and in some areas worse. It was just shocking. He became power drunk."

Mr Howard expressed deep frustration that South Africa had never put more pressure on its neighbour and called for its leader Jacob Zuma to push for democratic reforms.

"This is surely an opportunity for South Africa, if it had any sense of continuing responsibility, to intervene and argue the case for open and free elections," he said. "In my opinion the failure of South Africa over 20 to 30 years to do other than put a protective arm around Mugabe has perpetuated his misrule."

Asked whether he thought that was likely, he replied: "No I don't think there's much hope, certainly not with Zuma. I don't think there's a lot of hope of that at all but that doesn't mean it shouldn't be advocated."

Mr Howard was chairman of the Commonwealth between 2002 and 2003 around the time Mr Mugabe was forcibly seizing white-owned farms and driving his country into economic ruin.

"I had an animosity towards his deliberate dictatorial methods. It was about his violation of any semblance of democratic behaviour. There were many black citizens of Zimbabwe that were just as badly treated," he said.

He said he understood that South Africa shared with Zimbabwe an opposition against the history of apartheid.

"But there comes a point particularly when a country has become economically ransacked and driven into the ground as Zimbabwe has, something else has got to be given greater priority."

He said Thabo Mbeki, who was president of South Africa in the early 2000s, had "failed to call" Mr Mugabe out.

Mr Howard said the then suspension of Zimbabwe from the Commonwealth "seemed for some people a big symbolic gesture but it wasn't of enormous consequence".