An investigation is underway into the cause of a large fire that engulfed a power substation south of Wollongong, resulting in millions of dollars in damage. 

Around 3pm on Saturday one of four transformers at the Yallah substation failed, causing multiple explosions which started an oil fire.

Approximately 60 fire fighters responded to the incident, which occurred two kilometres from the Tallawarra power station.

They managed to contain the blaze and secure some 100,000 litres of oil.

The facility's operator, Transgrid, will investigate the exact cause of the mechanical failure.

"There will be an investigation process that any asset failure goes through," Transgrid's general manager of maintenance, Ian Davidson, said.

Fire crews remain at the scene monitoring a small fire which is continuing to burn.

Transformer a 'write-off'

Transgrid will begin the process of replacing the destroyed transformer.

"The unit that failed is definitely a write-off," Mr Davidson said.

"It will take a replacement program, civil rebuild [and] replacement of minor equipment surrounding it."

He estimated the replacement project would cost between $6 million and $10 million.

The Australian Energy Market Operator (AEMO) confirmed the fire had no impact on the electricity supply for NSW residents.

The three remaining transformers will continue to supply power while the damaged unit is replaced.

"Our substations have multiple units, so we have three other transformers that have the same capacity down here which will continue to supply the local area," Mr Davidson said.

"We will now begin to start the work that will replace that failed unit with one of our strategic spares."

The incident is not believed to be a result of the recent increase in electricity demand.