A worker has reportedly fallen into contaminated waste while cleaning up a large spill of uranium slurry at the Ranger mine in NT.

Mine operator Energy Resources of Australia (ERA) says the spill was contained soon after it occurred early last week, but it has not confirmed the more recent incident.

The Australian Manufacturing Workers Union (AMWU) says one man was buried up to his shoulders after falling through a thin crust on top of the slurry that spilt out of a collapsed tank.

The AMWU says the fitter was cleaned up and made to return to work, but refused.

ERA says it will look into the details and validity of the claims.

Meanwhile, the Federal and Northern Territory governments are undertaking a joint investigation into the spill, releasing its terms of reference late last week.

The probe will look at reasons the tank collapsed, examine the processing operations at Ranger, and look for any flaws in maintenance protocols. It will be conducted in consultation with traditional owners.