Power and gas provider AGL has lowered earnings guidance due to a fire which will keep its Liddell power station in NSW out of operation until March.

AGL on Monday said underlying profit for the financial year was expected to be between $500 million $580 million, down from a previous range of $560 million to $660 million.

A fire on Thursday ignited in the generator transformer during a change of oil cooler filter. The transformer was damaged and the unit had to be shut down.

An employee was seriously injured, the company said.

The cost of replacing the transformer and trading impacts to March would be $25 million, according to AGL.

The change in earnings guidance was also based on recent trading performance, and deteriorating conditions in the wholesale energy market.

Warmer winter weather and lower wholesale electricity prices have affected AGL’s business.

Shares were lower by 5.3 per cent to $12.52 at 1537 AEDT.

AGL shares have fallen more than 37 per cent since January 1.