The Australian dollar fell to an 18-month low against the US dollar on Monday, as worries about an impending financial crisis in Turkey prompted selling in the currency with strategists warning of more downside ahead.

The Australian dollar fell 1 per cent at the end of last week and stumbled again on Monday to trade at US72.83¢, a level not seen since the beginning of 2017.

The Australian dollar slumped below US73 cents this morning. Credit:Louie Douvis

"It's a stark reminder that the Australian dollar enjoys the status as the market's preferred risk proxy," National Australia Bank's head of foreign exchange, Ray Attrill, said.

There could be more weakness for the Australian currency to come, he said, with the signs "relatively ominous" from a technical point of view after the latest drop pulled it out of the trading range it had been in for the past year and a half.