THOUSANDS of tonnes of radioactive waste may stay buried under a property in a Sydney harbourside suburb if a suitable waste dump cannot be found.

The State Government yesterday admitted the full extent of the contamination beneath a vacant residential block owned by the Health Department in Hunters Hill would not be known until it is unearthed.

And it has also admitted that a cloud still hangs over how the waste, which includes uranium 238, thorium and radium is to be classified.

It claimed a decision had yet to be made on where it would be sent although the firm contracted to remediate the site told a local MP it would be dumped in a western Sydney landfill.

The startling admission comes after The Daily Telegraph contracted a company to investigate contaminated land in the Hunters Hill area.

A leading scientist from Monash University said the dumped uranium and thorium might have to be included in a national nuclear waste dump - yet to be built.

Dr Gavin Mudd, from the university's civil engineering department who chronicled a history of the Woolwich site dating back to 1911 when it was the site of a uranium ore processing plant, said it should be stored at the Lucas Heights nuclear research facility.

"Longer term studies would be needed about risk of elevated cancer rates . . . It should have been included in the plans for a national dump."