An environmental engineer says Energy Resources of Australia (ERA) should not have reopened its uranium mine within Kakadu National Park last month without committing to a waste water treatment plan.

The company had to stop processing uranium ore for five months during the Northern Territory wet season because its radioactive tailings dam got too close to capacity.

ERA says it plans to spend $80 million on a feasibility study to build a brine concentrator at the Ranger uranium mine by late 2013.

In the meantime, it will raise the dam wall.

Gavin Mudd from Monash University has been studying the environmental impacts of the ERA mine, which has been majority-owned by Rio Tinto for a decade.

He is worried the company is not planning to start processing its radioactive waste water for two more wet seasons.

Mr Mudd says a higher dam wall will mean the company will have even more radioactive water to process.

"In that sense it is still the temporary, sort of step-by-step measures," he said.

"We need decisive action on site to actually deliver large-scale process water treatment.

"Without that, the Ranger mine will continue to face risks every wet season with these types of issues and it will continually be compromising the ability to achieve rehabilitation [of the site] from 2021 at Ranger."

Dr Mudd says building the water plant should not depend on whether ERA expands its mine and processing plant.

"ERA need to solve the process water problem and so it should not be conditional on any expansion," he said.

"It should be absolute and the regulators need to put their foot down, as well as the board of Rio Tinto, from London and Melbourne.

"They need to dictate that this operation lifts its game."

ERA says its plans for a brine concentrator are "progressing well" and the plant is expected to be operating by the second half of 2013.