It followed an embarrassing incident at 1.24pm on Monday when an email was sent to key stakeholders advising that the planning minister had approved the controversial project.

A senior assessment officer issued an email shortly afterwards to explain that the initial email had been sent in error from the planning portal.

"There will be details formally announced in the coming days prior to Christmas," he wrote.

The department issued a statement late on Monday confirming that the project had been approved "following rigorous assessment and consideration of community feedback".

The circumstances surrounding the email bungle are unclear, but it comes on the same day as former Planning Minister Rob Stokes handed the portfolio over to incoming minister Anthony Roberts.

The department's approval, which was widely tipped to occur before Christmas, is the final regulatory hurdle for the project.

Construction of the plant is due to start on the site of the former aluminum smelter in the new year.

The embarrassing error triggered outrage from numerous groups that have been fighting the project over the past 18 months.

"The NSW Perrottet Government appears to have made an embarrassing error today, accidentally issuing a notification via the planning portal announcing the polluting Kurri Kurri gas plant had been approved, Lock the Gate NSW spokesperson Nic Clyde said.

"Instead, it appears the government intended to approve the controversial plant closer to Christmas, when fewer people would be watching.

"It's little wonder they were trying to hide it - most people in NSW would prefer to see renewable energy gifts under the Christmas tree, instead of more of the same polluting fossil fuel projects."

Gas Free Hunter Alliance spokeswoman Fiona Lee said the project should not be approved.

"Even the conservative International Energy Agency has said there can be no new gas projects if we are to reach net zero goals and try to avoid the increasingly catastrophic effects of climate change,"

"The local community deserves a clear plan from the Morrison Government to transition away from our dependency on fossil fuel employment, instead of lumping our community with this expensive out of date dinosaur that will only provide ten ongoing jobs at a cost of $600 million of public money."

The Federal Government has consistently argued the gas peaker is needed to ensure stability and affordability of electricity supply following the closure of Liddell Power station next year.

The project will firm the equivalent of 160,000 household solar installations, thereby reducing emissions by about 87 per cent compared to coal fired energy.

An environmental impact statement for the project released in May shows the plant will run for approximately two per cent of the time.

It is expected to create about 250 jobs during construction while there will be 10 ongoing full time jobs once complete.

Of the 261 submissions received during the recent public exhibition period, only two were in favour of the project.

The plant's two turbines, which can be modified to use of blend of hydrogen and gas, have been ordered from Japan.

The project will now be submitted to the Commonwealth for final approval.

The Federal Government hopes the generator will be ready to produce power by December 2023.

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