PLUNGING commodity prices have knocked Clive Palmer off Australia's top 50 rich list.

Mr Palmer's estimated wealth has fallen below $US550 million - the cut-off for the latest Forbes Australia list - as the value of his coal and iron ore assets fell.

He's not the only mining magnate whose fortune has been hit by tumbling commodity prices.

Gina Rinehart's wealth dropped by $US6 billion, but she remains the country's richest person with a net worth of $US11.7 billion.

Fortescue Metals founder Andrew Forrest dropped from fifth to 10th on the list, as his net worth shrunk by $US2.1 billion to $US5 billion.

Forbes said Australia's mining royalty had taken a hit from a worldwide decline in the demand for steel, particularly in China.

"The drop in iron ore prices and the Australian dollar's 6.5% depreciation against the US dollar were among the factors that impacted many of the richest," it said.

The collective worth of Australia's 50 richest people was $US89.3 billion in 2014, down from $US101.15 billion.

Sydney developer Harry Triguboff rose to second on the list as his fortune more than quadrupled to $US5.6 billion.