The world's reserve currency has gone from hero to zero in less than three months.

The US dollar Index has tumbled 6 per cent from its around 102 on March 20 to a three month low above 96, as the scramble for once scarce US dollars has petered out as the Federal Reserve has flooded the world with liquidity and investors have jettisoned safe havens like the greenback for riskier plays like the Australian dollar.

Not that US President Donald Trump will be complaining.

Having repeatedly railed against a strong US dollar - and weaker foreign currencies - as a bridle on US exports, the sliding value of the greenback provides a well timed tailwind as the world's largest economy slowly stirs back to life from COVID-19 lockdowns and the chaos of violent street protests.

Federal Reserve chairman Jerome Powell also won't be complaining as he keeps the pedal to the metal by keeping monetary policy ultra-accommodative and liquidity pumping in order to get the formerly $US22 trillion economy back to full capacity as quickly as possible.

Read the full story at afr.com.