Energy provider AGL has cut power to an NSW aluminium smelter as the state tries to manage record demand for electricity.
The Australian Workers Union has warned the move could "batter" the smelter but AGL says the cut was required to ensure power was kept on in "schools, homes and other small businesses".
The Tomago smelter uses 950 megawatts a day - about 12 per cent of the state's electricity - and is AGL's largest single customer.
The smelter's three pot line are being shut down sequentially for 75 minutes each from 3.45pm on Friday, Tomago Aluminium chief executive Matt Howell told AAP.
"Our concern is that this was done in the hottest part of the day when the market operator AEMO ... said they only needed us to curtail load for a maximum of three hours. That's one hour per pot line."
Workers have donned special suits and are labouring in 80C heat in the smelter on Friday evening to save the plant's equipment.
AGL defended the move saying it had "certain contractual rights" to interrupt electricity supply at the smelter.
"If power to the smelter is not curtailed, schools, homes and other small businesses will suffer a loss of power at the peak periods of demand this (Friday) afternoon," an AGL spokesman told AAP.
"This procedure has been implemented previously without adverse effect in similar circumstances to those that exist today."
But Mr Howell says it's incorrect to say there's been "no harm done" in the past.
The union argues workers will need to pull out all stops to save the plant's equipment and "they may not be successful".
An outage at the Portland smelter in Victoria in 2016 destroyed millions of dollars worth of equipment, AWU national secretary Daniel Walton said in a statement.