The Australian dollar is at a two-and-a-half-year low, having briefly dipped below 71 US cents on a rising greenback and US-China trade concerns.

The dollar hit a low of 70.99 US cents late on Friday's Wall Street session and again this morning. It has since recovered somewhat to be worth 71.09 US cents at 8:07am (AEST).

It is the first time the Australian dollar has traded below 71 US cents since February 2016.

NAB's head of foreign exchange strategy, Ray Attrill, said the Aussie dollar was the worst-performing major currency on Friday.

"AUD was Friday's standout loser, following by the NZD," he wrote.

"Contributing to its late-day weakness was the jump in USD-CNH (US dollar-Chinese yuan — to 6.87 from sub-6.85) … on the Trump quotes that he was prepared to add tariffs … so as to mean tariffs on the entirety of Chines imports."

The latest comments from US President Donald Trump were delivered to journalists during a mid-flight media briefing on Air Force One.

"The $US200 billion [in tariffs] we are talking about could take place very soon depending on what happens with them. To a certain extent it's going to be up to China," Mr Trump said.

"And, I hate to say this, but behind that is another $US267 billion ready to go on short notice if I want. That totally changes the equation."

The $US200 billion of tariffs that Mr Trump was referring to had already been announced in July, but were subject to a consultation period that ended last week, meaning they could now be imposed at any time.

This continued threat of an escalating trade war between Australia's main export customer and one of its other major trading partners is likely to mean further volatility ahead for the local currency.

"AUD broke below key technical levels on Friday and a test below 0.70 [US dollars] (2016 low) cannot be ruled out over the short term," noted ANZ's Cherelle Murphy.

"Eyes will remain on trade headlines with any negative impacts to China likely to hurt AUD."