The Australian currency opened weaker Thursday morning after ratings agency Moody's (NYSE:MCO) announced it would review the credit rating of global miner BHP Billiton (ASX:BHP).

At 7am east-coast time today, the domestic dollar was exchanging at 89.82 US cents, a decline from Wednesday's finish of 90.05 US cents. The Aussie was buying 76.74 yen, 69.9 euro cents and 57.6 pence.

Since 5pm, the local unit ranged from 89.72 to 90.47 US cents.

The Aussie fell overnight after Moody's announced it would review BHP's A1 credit rating if the miner starts raising large amounts of debt to complete the takeover of Canada's Potash Corp., according to Latitude FX sales analyst Jeremy Jukes.

"That's one reason the Aussie fell away," he said.

The ratings agency, however, said mining giant's credit rating would be unlikely to drop below A2 even if the deal went ahead.

According to Mr Jukes, the local unit had been prone to a fall after yesterday's release of the Westpac/Melbourne Institute's index of economic activity, which suggested a slow down in the Australian economy over the next nine months.

The survey's leading index of economic activity, which is closely watched by market observers, was virtually unchanged in June after it shed 0.1 point.

On an annualised basis, the growth rate slowed down to 6 per cent from 7.4 per cent in May.

With no market-moving data expected during the domestic session, currency traders would take their lead from the local bourse, Mr Jukes said.