Donald Trump "emphatically" promised to exempt Australian steel and aluminium from US tariffs during a meeting with Prime Minister Malcolm Turnbull last year, it can be revealed.

Key points: Mr Trump and Mr Turnbull had a meeting last year in the so-called "Steel Cage"

Mr Trump and Mr Turnbull had a meeting last year in the so-called "Steel Cage" The US tarrifs would impose a 25 per cent tax on imported steel and 10 per cent for aluminium

The US tarrifs would impose a 25 per cent tax on imported steel and 10 per cent for aluminium Mr Trump promised Australia would be exempt during a meeting last year

The ABC understands the promise was witnessed by high-ranking officials on both sides of the meeting, which was held on the sidelines of the G20 meeting in Hamburg, Germany, in July 2017.

Among those in the US delegation who saw the undertaking first-hand were US Treasury Secretary Steven Mnuchin, Commerce Secretary Wilbur Ross and White House Chief Economics Adviser Gary Cohn.

On the Australian side were Finance Minister Mathias Cormann and the Deputy Secretary of the Department of Prime Minister and Cabinet David Gruen.

This revelation explains why the Australian Government has been stunned by Mr Trump's declaration last week that the tariff regime will be enforced, and subsequent statements by Mr Ross that country-specific exemptions are unlikely.

The conversation between the President and the Prime Minister was in the so-called "Steel Cage", a secure communications pod that travels with the US President.

Sources have told the ABC Mr Trump's promise was emphatic and that he instructed Mr Ross to work out the specifics to "make it happen".

The Prime Minister and the Australian delegation was "absolutely certain" that a deal had been struck during the Hamburg meeting.

And having secured the assurance, the Turnbull Government dispatched Ambassador Joe Hockey to follow it through with the Trump administration in Washington.

Donald Trump and Malcolm Turnbull met at the G20 summit last year and the White House just before the tariffs announcement. ( AP: Pablo Martinez Monsivais )

The US tariffs, which have alarmed major exporters such as BlueScope, would impose a 25 per cent tax on imported steel and 10 per cent for aluminium.

Australia exports about $500 million in steel and aluminium to the US a year.

Federal Government officials have been increasingly pessimistic about Australia's chances of dodging the US steel and aluminium tariffs unveiled by Mr Trump.

Australian Trade Minister Steven Ciobo had been concerned about the tariff. ( AAP: Mick Tsikas )

Mr Turnbull and Trade Minister Steve Ciobo have been lobbying the Trump administration to spare Australia from the tariffs.

Mr Ciobo has been urgently trying to clarify whether Australian steel and aluminium will be hit.

"My concern remains that off the back of actions like this, we could see retaliatory measures put in place by other major economies. That's in no-one's interests. Let's be clear," Mr Ciobo said last week.

Opposition Leader Bill Shorten said he hoped Mr Turnbull would get on the phone and "at least try" to secure an exemption for Australia.

Mr Shorten presented the issue as a litmus test for the US alliance.

"Australia and America have been allies in all sorts of adversity and conflict. I hope at times like this that all of our other friendship counts for something," he said.