Owner Energy Resources of Australia announced processing of stockpiled ore finished on Friday.

ERA CEO Paul Arnold described the end of processing as a "truly historic milestone".

"Ranger has been a major supplier to global energy markets as well as being a key contributor to the Kakadu region and the Northern Territory," he said.

"On behalf of the Board and management of ERA, it is with heartfelt gratitude that we farewell many of our dedicated operational team who have contributed both to Ranger and the broader Jabiru and Kakadu community.

"We would like to thank the many generations of ERA employees who devoted their skills and expertise throughout the life of this mine.

"Importantly, ERA will have an ongoing presence in the region as we continue the progressive rehabilitation of Ranger."

The mine produced 390 tonnes of uranium oxide in the December quarter and 1574t for 2020, at the upper end of guidance of 1200-1600t.

Ranger was opened in 1981 and has operated continuously ever since, producing an estimated 132,000t of uranium oxide.

Open pit mining ceased in November 2012 and the operation has been processing low-grade stockpiles ever since.

An underground operation, Ranger 3 Deeps, had been considered, but faces "material barriers to development", given the weak uranium market.

ERA plans to also close the Ranger 3 Deeps decline this year.

The company's focus will now turn to the ongoing rehabilitation of the mine, 8km each of Jabiru.

More than A$640 million has been spent on rehabilitation since 2012.

The transfer of tailings from the tailings storage facility to Pit 3 is continuing, with bulk dredging works forecast to be complete later this month.

ERA continues to monitor the rate of tailings consolidation in Pit 3 compared to the consolidation model assumed for the purposes of the rehabilitation feasibility study.

Cone penetration testing to assess the extent of tailings consolidation is progressing with further studies being undertaken to inform the detailed engineering design for Pit 3 wicking, capping and bulk backfill works.

TSF floor cleaning activities will occur through the first half of 2021. Other key rehabilitation activities continued including water treatment and Pit 1 revegetation.

Decommissioning of the plant has already begun.

The rehabilitation process is expected to cost about $925 million and must be completed by January 2026.

ERA raised $476 million early last year to fund the process, with major shareholder Rio Tinto increasing its stake from 68.4% to 86.3% as part of an underwriting agreement.

Australian Conservation Foundation nuclear campaigner Dave Sweeney said the end of processing was a good day.

"The Ranger mine has generated controversy, headlines and heartache for four decades," he said.

"The focus must now be on ERA and parent company Rio Tinto doing comprehensive and credible site rehabilitation and supporting the transition to a post-mining regional economy.

A recent report co-authored by ACF found data deficiencies and technical issues, particularly around groundwater and tailings management.

"Australia has a long history of sub-standard mine rehabilitation in the uranium and wider mining sectors. A far better approach and outcome is needed at Ranger," Sweeney said.

"This work is a key test of the commitment of ERA and Rio Tinto, as well as the Northern Territory and federal governments."