Australian Prime Minister Malcolm Turnbull said he would negotiate a "very strong" free trade agreement with Britain after it leaves the European Union, a major boost for Theresa May who hailed the "long-standing ties" between the two nations.
Mr Turnbull’s comments at the G20 meeting of world leaders in China comes after Japan and the United States had issued warnings to Britain over its status as a trading partner when it leaves the EU.
Australia was previously widely expected to be among the first countries to sign an agreement with Britain as Mrs May seeks a new global economic role for the UK.
Speaking after a bi-lateral meetings, Mr Turnbull said that Britain and Australia are "great friends" as he pledged to provide "all the support and assistance that we can" with Brexit.
He said: "Britain's made a very momentous and historic choice to leave the European Union and we have already been engaged in discussions with you about what the free trade arrangements may look like after that.
"Australia is determined to provide Britain with all the support and assistance that we can. We are such great friends, such strong allies. (There) couldn't be two countries with closer bonds."
Mrs May said: "It is a historic decision for British people. It will be a complex and challenging process leaving the European Union. "But I'm very clear that it doesn't mean that we are going to be inward-looking.
"In fact, we want to be even more outward-looking around the whole of the world. And obviously, Australia, with our longstanding ties and our close relationship, will be one of the first countries will be looking to."
Britain’s decision to leave the EU in a June referendum sent shockwaves around the world’s financial markets.
British Prime Minister Theresa May (2-R) meets Australian Prime Minister Malcolm Turnbull (2-L) CREDIT: EPA/POOL
Although recent economic data has indicated the British economy has not suffered as much as some predicted, the process of tearing itself away from the 28-country European bloc is likely throw up considerable challenges.
Japan on Sunday warned Britain that its banks, car manufacturers and pharmaceutical companies may leave the UK for Europe if Brexit leads to the loss of tariff-free trade.
A Government task-force published a 15 page list of demands from companies and warned that Britain may become an increasingly "unattractive" place for business.
It came as Barack Obama said he believes Britain was wrong to vote to leave the EU and suggested it is still at the "back of the queue" for a new trade deal.
The US President used a joint press conference with Theresa May to warn that while he does not want to "punish" Britain there is a risk that the trading relationship could "unravel".