French President Emmanuel Macron on Sunday said he was certain that Australian Prime Minister Scott Morrison lied to him over a cancelled submarine deal that triggered a major diplomatic crisis.

A journalist with Australia media caught Macron on the sidelines of the G20 summit in Rome, and asked him whether he thought the Australian leader had been untruthful to him. 

"I don't think. I know," Macron replied.

The journalist posted a video of the exchange on Twitter, in which the French president was also heard stressing the need for mutual "respect" and adding, "you have to behave in line and consistently with this value".

The row, pitting Paris against Washington and Canberra, erupted last month after Australia walked out of a multibillion-dollar submarines deal with Paris in favour of an alternative one with the US and Britain.

An enraged French government recalled its ambassadors from the US and Australia, and called the deal, which led to the cancellation of a contract with Australia worth more than €50 billion, "a stab in the back".

Macron crossed paths with Morrison at the G20, but spoke on the phone to him earlier this week and told him that a "relationship of trust" had been broken between France and Australia. 

In Rome, the French leader seemed to have made more progress in clearing the air with US President Joe Biden. 

At a meeting on Friday in front of journalists, Biden admitted to his French counterpart that Washington had been "clumsy" in the way it handled the deal, and said, "we have no better ally than France".