FERTILISER maker Incitec Pivot says the shutdown of a Mt Isa sulphuric acid plant for up to a month could cost it $25 million.

The waste heat boiler at the Queensland plant failed on Friday, and the plant may need to be offline for up to a month in order for repairs to be made, Incitec Pivot said today.

The closure of the Mt Isa plant will cause reduced operations at the ammonium phosphate plant at Phosphate Hill, south of Mt Isa.

That could reduce the plant's production of ammonium phosphates in the year to September 30, 2013 to 900,000 tonnes, the company said.

If the Mt Isa plant is offline for a month, the financial implication of higher costs of sulphuric acid and repairs is estimated to be in the region of $25 million, before tax, Incitec Pivot said.

IPL shares were down eight cents, or 2.6 per cent, at $2.99 at 10.29am.