THE uranium miner Energy Resources of Australia is seeking to at least maintain and possibly boost production at Ranger, the largest uranium mine in Australia, with the potential development of the Ranger 3 Deeps underground mine.

ERA has defined a resource of 34,000 tonnes at Ranger 3 Deeps. This makes it a similar size to the largest undeveloped projects in the country, such as the Kintyre project in Western Australia, which ERA's parent, Rio Tinto, sold to Cameco and Mitsubishi for $US495 million ($600 million) last year.

"This is one of the largest discoveries in the last 20 years anywhere in the world," ERA's chief executive, Rob Atkinson, said in a conference call with analysts after reporting first-half underlying earnings of $127.5 million. This figure was nearly quadruple the $33.9 million reported in the same period last year.

ERA has received the necessary approvals to build an exploration decline to further define the resources at Ranger 3 Deeps, but it has yet to finalise a study and therefore has not released the projected costs to the market.

Mr Atkinson said that his company was "several years" away from mining at Ranger 3 Deeps and its heap leach project would be completed beforehand to help it maintain its annual production of 5000 to 5500 tonnes of uranium in the meantime as grades drop in its existing open pit.

The mining lease at Ranger expires in 2021, and Mr Atkinson said there were no plans to mine beyond 2020. But the ground remains prospective for the discovery of more deposits, and ERA plans to spend nearly $30 million on exploration this year.

The market hopes ERA will be able to boost its production beyond existing levels based on the discovery of Ranger 3 Deeps, rather than simply extending the life of its present operation.

ERA received an average of $US48 a pound for its uranium in the first half, which represented a larger gain from earlier periods as older contracts were completed.

ERA shares closed 35c higher at $25.30.