* Plant cuts power to conserve electricity

* Comes as heatwave grips much of Australia

SYDNEY, Feb 10 (Reuters) - Australia's Tomago aluminium smelter has been forced to cut its power to conserve electricity as a heat wave grips much of the country, a government official said on Friday.

The Tomago smelter, which exports to Southeast Asia, Japan and China, is the single largest consumer of electricity in New South Wales, Australia's most populous state.

"Tomago are required by law under their contract to cut their power in instances like this," New South Wales state energy and utilities minister Don Harwin told reporters.

It was too early to determine the full impact of the power cut on Tomago's shipments and production.

The smelter located 150 kilometres (90 miles) north of Sydney and capable of producing more than a half-million tonnes of aluminium annually, is jointly owned by Anglo-Australian group Rio Tinto (RIO.AX) (RIO.L) and Oslo-based Norsk Hydro (NHY.OL) .

Harwin said Tomago Aluminium was "playing a big part" in conserving the state's power by taking its potlines -- the main source of electricity consumption -- off line as requested.

Smelter officials could not be immediately reached for comment.

An extreme heatwave has led to a spike in power demand, straining electricity supplies and causing outages in many parts of Australia.

A power outage last year at another smelter in Australia, owned by Alcoa (AA.N) , caused molten aluminium to solidify, forcing it to run at less than a third of capacity and threatened its future.

Alcoa waited until last month to begin work to reactivate the lost capacity after the government provided a $182 million rescue package.

Tags：UPDATE 1-Australia's Tomago aluminium plant cuts power due to heat wave