Police are preparing to mersin escort a monolithic steel cask of nuclear waste to Sydney this weekend, reigniting debate about Australia's plans for the toxic material.

The hulking capsule resembling something from NASA's space program contains two tonnes of intermediate-level radioactive waste that will need to be isolated from the environment for thousands of years.

But for the time being it will be stored at the Lucas Heights nuclear reactor compound in southern Sydney.

The waste is being returned under the international principle that countries must take back their nuclear leftovers after reprocessing.

In Australia's case that's been done offshore.

Conservation groups have been tracking the ship carrying the cask since it left the UK in January and expect it to be unloaded at Port Kembla between 11am and 11pm on Saturday.

It will then be trucked - under the highest security, along closed roads - to Lucas Heights.

It will be no small operation. The cask itself weighs 100 tonnes and has been built to withstand an earthquake and a jet strike.

The move is expected to be carried out overnight on Saturday, or Sunday.

At Lucas Heights the cask will be stored alongside one that was returned in 2015, with 20 tonnes of intermediate-level inside. Low-level waste is also stored at the reactor compound.

Down the track, all the waste will be moved to a new national nuclear waste facility that's yet to be built near the South Australian town of Kimba.

But some conservationists have a big problem with that plan.

Kimba will be a near-surface facility and a permanent solution for low-level waste only.

The intermediate material will once more be in storage.

The federal government has committed to developing a separate end solution for the more toxic stuff. It will involve deep burial but so far there's no firm plan, and no site has been identified to take it.

Australian Conservation Foundation campaigner Dave Sweeney says the nation's most potent nuclear waste should not be moved to Kimba.

He says the problem is being kicked down the road, for some future government to sort out.

"We believe there's a very real risk that this material gets stranded in sub-optimal conditions at Kimba. Move it once, move it well, and move it permanently," he says.

"Our position is that the Lucas Heights facility is the best place for Australia's most serious waste. It has the highest security, the highest emergency monitoring and response capacity. It is staffed 24/7, and 95 per cent of the stuff is already there."

The Australian Nuclear Science and Technology Organisation operates the Lucas Heights reactor, which supports nuclear medicine and science.

Resources and Water Minister Keith Pitt said it was international best practice to consolidate radioactive waste at a single, safe, purpose-built facility.

"That is what the government is delivering," he said, while noting it would take several decades to find an end solution for intermediate waste.

He said ANSTO had warned it would need to build three additional waste storage buildings at Lucas Heights if the national facility wasn't built.

For security reasons, ANSTO won't confirm when the cask will be moved from Port Kembla to Lucas Heights.

It said the cask is so well shielded that someone could stand next to it for 25 hours and get the same radiation dose as a nine-hour flight to Singapore.

Police have told AAP an operation is planned for Saturday to aid the transportation of cargo to ANSTO's Lucas Heights campus.

It said no further details would be provided.