Energy Resources Australia says water in the pit of its Ranger Uranium mine near Jabiru has caused a 22 per cent fall in production over the last three months.

ERA produces a tenth of the world's uranium and says in its June quarterly report it produced 1,030 tonnes of uranium oxide in the last quarter.

The report shows a 22 per cent drop in production from the previous quarter and a 31 per cent drop from the corresponding three months last year.

ERA is undergoing a major expansion of the mine and has spent $8 million on exploration and evaluation so far this year.