By mid-afternoon, AEMO reissued a Lack of Reserve level 1 alert for 4.30pm to 9pm, local time. The capacity requirement was 1550 megawatts, with available reserve at 771 megawatts.

AEMO said this was partly due to heavier than expected cloud cover which reduced the output from solar rooftop generation, resulting in increased demand from the grid.

Mr Howell dismissed media reports that AGL, its main supplier, was to blame for the shortfall, noting the problem was with the overall market.

Even so, AGL has had difficulties with its two Hunter Valley coal-fired power stations of late.

Bayswater is understood to have been down to just one of its four generation units after scheduling one of them for maintenance. Its nearby sister plant, Liddell, was operating at about half of its capacity.

Another plant struggling this week has been the Vales Point coal-fired power station, also in NSW, which had one of its two units trip on Thursday. The plant is owned by Sunset Power International.

"We have one unit currently out," a Vales Point spokesman said. "It'll come back online in the not too distant future."

Fairfax Media has approached Don Harwin, the NSW energy minister, for comment.

“Due to confidentiality AGL is unwilling to discuss its contract with Tomago," an AGL spokesman said.

“We can confirm a significant number of coal plants are unavailable across the NSW market and there is reduced capacity to meet higher winter demands.”

Mr Howell said the planned and unplanned outages of coal-fired generators had left NSW's power supply at the mercy of the weather.

“When the sun is shining in the middle of the day and the wind is blowing there is generally sufficient capacity," he said in the statement.

"When we need it the most - early mornings and evenings in summer and winter, our solar resources are useless, and the nature of wind resources frequently means that many wind assets are idle."

'This is the likely future'

Tomago was forced to curtail operations at the height of a heatwave in February 2017, leaving its potlines close to failure. If left to cool for too long, the molten metal solidifies, ruining the potline.

Since then, Victoria's Hazelwood power plant has closed and although much more renewable energy has been added to the network, dispatchable supplies can be strained during calm days or when the sun isn't shining.

“This is not summer with extreme demand. This is the likely future of our energy grid as once reliable baseload generators exit the [NEM] and are mostly replaced with intermittent wind and solar projects with no practical storage to speak of," Mr Howell said.

“Our energy debate should not advocate either renewables or conventional thermal," he said. "As with most things in life, we need a balance and we have reached a point where urgent rebalancing is required.”