Electricity load shedding in South Australia last week, which affected 90,000 customers, was three times the level ordered, an official report from the Australian Energy Market Operator (AEMO) has revealed.

Power distributor SA Power Networks said it was now looking into why automated load shedding software failed to work correctly.

Thousands of electricity customers were without supply last Wednesday evening for up to 40 minutes on a day of extreme temperatures when power demand outstripped power supply.

AEMO said the supply restriction was the only option to prevent a risk of prolonged damage to infrastructure.

In its preliminary report into the brief rolling blackouts, the AEMO said it issued a directive shortly after 6:00pm to transmission provider ElectraNet to reduce load by 100 megawatts.

ElectraNet in turn instructed local power distributor, SA Power Networks, to begin load shedding by switching off supply to designated areas.

But the report found that by 6:20pm, about 300 MW of electricity had been switched off, 200 MW more than the AEMO had demanded.

It said it was working with SA Power Networks to determine why more power load than asked was shed.

Load shedding ordered for total of 27 minutes

The report said AEMO gave a directive to restore the power load 27 minutes after its initial order.

Load shedding occurred because record power demand during heatwave conditions outstripped the available supply.

That demand put extra pressure on both the Heywood and Murraylink interconnectors between SA and Victoria.

At the time the load shedding was ordered, both were importing more power to SA than allowed under their operating limits.

AEMO said at the demand peak, wind generation was lower than forecast as was thermal generation due to forced outages at South Australia's Torrens Island, Quarantine and Port Lincoln generators.

In a statement, SA Power Networks said automated load shedding software had kicked in at a time of "some of the highest electricity demands ever recorded in the state".

"Within 10 minutes our network controllers raised concerns that more load was being shed than the 86 MW the load-shedding system was indicating."

It said steps had been taken to prevent a recurrence and a detailed investigation was continuing into why load shedding software failed to operate correctly.

South Australian Treasurer Tom Koutsantonis slammed AEMO last week for choosing not to turn on a second generation unit at Pelican Point in north-west Adelaide, saying it got its demand forecast wrong.

On Wednesday, he said AEMO needed to apologise to South Australians.

"This report proves once and for all that South Australia reached its highest ever demand on Wednesday when the national operator decided to shed customers," he said.

"But it also showed that we had enough local generation to meet that record demand but it wasn't turned on."

AEMO says energy demand issues complex

AEMO executive general manager Joe Adamo said there needed to be a more united approach to managing Australia's energy needs.

"The complexities and challenges of managing short-notice generation capacity reductions amid high temperatures and increasing electricity consumption are real," he said.

"Energy transition needs co-ordinated planning and this is best achieved when we are all dealing with facts.

On the failure to turn on a Pelican Point gas generator in the heatwave conditions, the report said 165 MW of generation capacity at Pelican Point had been notified as "unavailable".

It said that at 5:39pm, about 25 minutes before load shedding occurred, AEMO sought advice from the Pelican Point owner Engie on the availability of the generating unit.

AEMO said Engie advised it did not have the gas to run the unit and, if gas was available, it would be a four-hour minimum run time.

At 6:01pm, two minutes before the load shedding was ordered, Engie informed AEMO that, if directed, the offline unit could be available to synchronise by 7:00pm and be at full output by 7:45pm for a four to eight-hour run time.

AEMO said it decided the Pelican Point unit would not be available in time to boost the power system's security.