"The improvement of the water level of Lake Toba was the main reason for the increase (this year)," an official for the company said. The lake in northern Sumatra is the main source of water for the smelter's hydroelectric power plant.

PT Indonesia Asahan Aluminium, also known as PT Inalum, produced 163,000 tonnes of aluminium last fiscal year as a result of low water levels that the company attribued to the El Nino weather phenomenon.

PT Inalum is owned 59 percent by Nippon Asahan Aluminium Co Ltd, a 12-company Japanese consortium that counts among its members Sumitomo Chemical Co Ltd (4005.T: Quote, Profile, Research) , Nippon Light Metal Co Ltd (5701.T: Quote, Profile, Research) , Sumitomo Corp (8053.T: Quote, Profile, Research) , Mitsui and Co Ltd (8031.T: Quote, Profile, Research) , Marubeni Corp (8002.T: Quote, Profile, Research) and Mitsubishi Corp (8058.T: Quote, Profile, Research) .

The government of Indonesia holds the other 41 percent of PT Inalum.

The official said the smelter sells around 60 percent of its metal output to Japan and 40 percent to the domestic market in Indonesia, most of it on a long-term contract basis.

PT Inalum's nameplate capacity is 225,000 tonnes a year and the company said it had no plans to expand.

It said it received its alumina, the key raw material for producing aluminium, from Australia.

Benchmark three-month London Metal Exchange aluminium was trading at $1,645/1,648 a tonne by 0910 GMT yesterday. The metal hit a 6-1/2-year high of $1,775 in mid-February.