The former Australian prime minister Tony Abbott has raised fears Beijing “could lash out disastrously very soon” amid growing tensions over the future of Taiwan – and argued the US and Australia could not stand idly by.

Delivering a keynote address to a regional forum in Taipei on Friday, Abbott dismissed claims that Australian officials were beating the “drums of war”, while calling on Beijing to “scale back the aggression”.

Abbott is visiting Taiwan as a private citizen, but the conservative former prime minister was granted an audience with Taiwan’s president, Tsai Ing-wen, on Thursday.

Abbott used an address to the Yushan Forum on Friday to accuse China of displaying “growing belligerence to Taiwan” – including through a recent increase in incursions by military aircraft into Taiwan’s air defence zone.

“Sensing that its relative power might have peaked, with its population ageing, its economy slowing, and its finances creaking, it’s quite possible that Beijing could lash out disastrously very soon,” said Abbott, who was prime minister from 2013 to 2015.

“Our challenge is to try to ensure that the unthinkable remains unlikely; and that the possible doesn’t become the probable.”

Abbott said he did not believe the US “could stand by” and watch Taiwan be “swallowed up” by China.

“I don’t think Australia should be indifferent to the fate of a fellow democracy of almost 25 million people,” he added.

Abbott – the prime minister who signed Australia’s free trade agreement with China in 2014 – said “much has changed” since then. But he said Australia continued to have “goodwill towards the people of China, about a million of whom are now Australians and making a fine contribution to our country”.

“Australia has no issue with China – we welcome trade, investment and visits, just not further hectoring about being the chewing gum on China’s boot,” Abbott said.

The secretary of Australia’s Department of Home Affairs, Michael Pezzullo, attracted criticism in April when he said that free nations “again hear the beating drums” of war, and were bracing for “the curse of war”.

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In Friday’s speech Abbott said: “So if the ‘drums of war’ can be heard in our region, as an official of ours has noted, it’s not Australia that’s beating them. The only drums we beat are for justice and freedom – freedom for all people, in China and in Taiwan, to make their own decisions about their lives and their futures.”

Abbott said China had taken “a wrong turn”. Its actions were responsible, he said, for the reinvigorated Quad grouping of the US, Japan, India and Australia.

“The more aggressive it becomes, the more opponents it will find,” Abbott said.

Abbott said he had hesitated to attend the same conference two years ago “lest that provoke China” but he cited a range of developments since then, including the clampdown on dissent in Hong Kong and “weaponising” trade against Australia.

In an apparent reference to the Chinese president, Xi Jinping, Abbott said Beijing had “cancelled popular personalities in favour of a cult of the red emperor”.

Abbott called on governments – including Australia’s – to welcome Taiwan into the trade deal now known as the Comprehensive and Progressive Agreement for Trans-Pacific Partnership (CPTPP).

Abbott also suggested China’s own bid to join the CPTPP should be blocked at the current time. He said China could not be admitted to the trade grouping “while engaged in a trade war with Australia, and in predatory trade all-round”.

The former prime minister said China could “hardly succeed while it mistreats its own people and threatens its neighbours”.

“Nothing is more pressing right now, than solidarity with Taiwan, if we want a better world; hence my enthusiastic presence here today, to stand with this island that’s brave and free,” Abbott said.

“I won’t end urging you to stay safe rather something nobler and higher: stay free.”

Tsai spoke immediately before Abbott at the forum on Friday, telling the audience she had enjoyed their conversation the previous day.

Australia’s foreign affairs minister, Marise Payne, said the government was committed to its one-China policy – but that did not prevent Australia from strengthening ties with Taiwan, which she described as a “leading democracy” and a “critical partner”.

“We have been concerned by tensions across the Taiwan Strait sharpening in recent months,” Payne told the ABC on Thursday. “It is clear that conflict is in no one’s interests here and we are concerned by increased air incursions by China into Taiwan’s air defence zone in the past week.”

Comment has been sought from the Chinese embassy in Canberra.