Australia's trust in the United States has halved since 2011 and President Donald Trump is unpopular with 60 per cent of people, but most still think the alliance is important.

The 2017 Lowy Institute's poll, released today, shows continuing support for the relationship is driven by growing concerns about China's military threat.

The institute's director of polling, Alex Oliver, said despite diminishing trust in the US, Australians believed the alliance "has served us well" and could safeguard against future threats.

"We have persistently seen China as a possible military threat, and I think Australians see the alliance as an insurance policy against a potential threat in our region, including possibly from China," Ms Oliver said.

Nearly half of the 1,200 people polled said it was likely "China will become a military threat to Australia in the next 20 years", but far more — 79 per cent — saw the nation as a vital trading partner.

"The fact that Australians are relatively optimistic economically, that we still see globalisation as good and free trade as good, is largely because we have had this tremendously prosperous arrangement with China, which has propped up our economic performance in the world," Ms Oliver said.

"The balance clearly comes out in favour of the economic relationship, so the military threat would have to be pushed very, very far in order for us to put the economic relationship in jeopardy."

The poll shows the US and China tied when Australians were asked which relationship is more important, with younger people leaning toward China and older Australians favouring the US.