Australian politicians have welcomed the flamboyant UK leader, praising his "unusually close" relationship with Australia, after he was elected as leader of the UK Conservative Party and elevated to prime minister-designate overnight.

But an expert in British politics cautions that the UK - and its new prime minister - could quickly be headed into a general election, depending on how the Brexit plans play out.

"I can't really see any way that they're going to avoid a general election," University of Sydney professor Simon Tormey told AAP on Wednesday.

"I think Australians are going to have to sit tight and wait to see how this political crisis plays out, to see to what extent there's actually going to be any impact in terms of British-Australia relations."

Liberal senator James McGrath - a senior staffer to Mr Johnson when he was London mayor - believes his old old boss will be good for Australia.

"He's been to Australia quite a few time, he's a friend of Australia," Senator McGrath told Sky News.

"He's prime minister now of our closest ally; it is good news for Australia."

Former foreign minister Julie Bishop says Boris Johnson has an "unusually close" relationship with Australia, and expects this bond to become even tighter.

"He spent his gap year here, attending Geelong Grammar. He's visited Australia many times," she told the Seven Network.

"He was here for the annual Australia-UK ministerial forum. I hosted him in Sydney and he spent several days here reminiscing about the good times he's had in this country."

Prime Minister Scott Morrison congratulated Mr Johnson via Twitter, saying he expects to meet him when they both travel to France for the G7 in August.

Prof Tormey sees an opportunity for Australia in the chaos around Brexit, with officials poised to negotiate a new trade deal as soon as the UK leaves the European Union.

The British need that deal more than Australia does, he says, with the very best they could hope for being that Australia rolls over the same arrangements it now has in place with the EU.

"I think Britain will actually have to cough something up in order to maintain the already very good trade agreement it has as being part of the EU," he said.

"Australians have actually had quite a difficult time with the British visa regulations and I think what the Australians are likely to do, what they should be doing, is extract a better visa regime for Australian citizens who want to go and work and live in the UK."

Trade Minister Simon Birmingham said he was working to "make sure that the Australia-UK relationship yields the maximum possible benefits for the people to both our nations".

Australian Associated Press