The damaging storms that tore through parts of NSW have left Insurance Australia Group facing a $300 million bill.

The insurer behind NRMA Insurance and CGU says last week’s heavy rain storms in New South Wales are likely to cost it $250 million as it pays out claims to customers.

Last weekend’s sudden downpour of hail in Sydney is also likely to leave the general insurer with a $50 million bill.

The storms have helped blow out the amount IAG expects to pay in natural peril claims this year from $700 million to $1 billion.

As a result, IAG expects its full-year insurance margin, a key measure of its profitability, to be between 10.5 per cent and 12.5 per cent, down from its previous forecast of between 13.5 per cent and 15.5 per cent.

IAG shares were down 24.5 cents, or 4.2 per cent, at $5.645 as of 1030 AEST.