Mr Corbyn and his cohorts were "totally hostile" to the traditions of Western security policy, and unsympathetic and hostile to Western interests, he said. He had made friends and allies of the likes of Hamas, Hezbollah, Iran and Venezuela and as a result Australia would have to be "very careful" about the sort of intelligence it shared "with a government like that", Mr Downer said, speaking at the Press Club in Canberra.

Mr Downer refused to answer questions on the United States president Donald Trump's investigation into the origins of the Mueller inquiry into Russian meddling in the US election, in which Mr Downer played a pivotal role. He wouldn't say whether he is meeting foreign affairs officials while in Canberra in relation to the inquiry, but said he was fully cooperating.

He predicted a 70 per cent chance that Boris Johnson's conservatives would win on December 12, but if said if Mr Corbyn's Labour party won government he would run a "Venezuelan-style economic agenda", sparking a very substantial deterioration in the British economy.

Mr Corbyn would abandon Britain's commitment to the Western security alliance and Australia would be unwise to continue its intimate intelligence-sharing relationship under a government he led, Mr Downer said, characterising Mr Corbyn as "something we have never seen, way over to the very very far left".

Mr Downer was not specific, but in Mr Corbyn was reported to have sent a message of support to a London march in May, condemning Israel's human rights abuses in Gaza - a message that was "saluted" by Hamas. Mr Corbyn has also opposed sanctions on Venezuela.

Mr Downer, who noted that his father was high commissioner to London when Britain joined the European Union and he had book-ended that as high commissioner when Britain voted to leave the union, said there were upsides for Australia in Brexit.

The British establishment had shown scant interest in Australia, with not one visit from British foreign minister to Australia in the 12 years Mr Downer had been foreign minister.

Outside the European Union, Britain would look to countries with historic links, and Australia should be able to negotiate a free-trade agreement. Such an agreement could be concluded quickly, in place by 2021, he said. It might allow Australia to sell more sugar and perhaps sheep and beef into Britain, but the big gains would be mutual recognition for professionals, such as lawyers, doctors and architects, and perhaps a better visa deal.

The European Union, though, would be more difficult for Australia to deal with without Britain, with the EU already seeing Australia as a "bit player" and likely to be less interested still.

Mr Downer said Britain's exit would be a huge blow to the EU, given it was the second biggest economy in EU, welded the second largest amount of soft power in the world behind the US, and had the largest and most effective defence force in the EU and was prepared to use it.

The EU would be weaker, more introverted and less transatlantic, and would play a smaller role in world affairs.

Mr Downer also had some advice for Australia on China, saying under John Howard no-one had been allowed to "bully" Australia.

Australia should be very clear about its parameters in the relationship, and speak out when China did things with which it disagreed.

"China needs to remember that this is a relationship of mutual benefit. We are not going to China begging for forgiveness for something that Malcolm Turnbull may have said during a by-election," he said. "We need to be more self-confident. We're not going to kowtow ... to anybody. And not to China."

China depended very heavily on Australia for iron ore, coal and agricultural products and could not drive its economy without secure supplies such as those from Australia.

"They need to get on with us," he said. "Not just we need to get on with them. This is a relationship of mutual benefit."

It was important that universities resisted China's attempt to influence staff and teaching, he said, supporting Australia's decision to block Huawei from bidding to roll-out the country's 5G network.

China would not accept Telstra being involved in similar infrastructure in China. It had constrained Google and banned Facebook and Twitter. Instead of complaining about being excluded from critical infrastructure in Australia, it should "look in the mirror", Mr Downer said.

As for the computer hacks into computer systems at the Australian National University and Parliament House, attributed to China but not confirmed, Mr Downer said, "Launching cyber-attacks on Australia is not going to go down well with Australians. It's as simple as that."