NATIONALS MP Kay Hull is to retire at this year's federal election after 12 years in federal parliament representing the NSW seat of Riverina.

Mrs Hull, 56, advised her party of her decision yesterday, promising "business as usual" between now and the election, The Australian reports.

Sources said last night that Mrs Hull would be the only sitting Nationals MHR not to seek another term in parliament and that a strong field was expected to nominate for the party's endorsement.

The Nationals chief whip held the seat at the last election by a margin of 16 per cent.

"I will, of course, leave no stone unturned in order to have the selected Nationals candidate win the seat of Riverina," she said.

Nationals Leader Warren Truss last night described Mrs Hull as one of the nation's hardest-working politicians.

"Many people in work today in the Riverina owe their jobs to Kay as she has worked hard to provide opportunities for regional development, small business, health, aged care and the disabled," Mr Truss said.

"Kay will be incredibly difficult to replace, but the search is now on to find a new candidate."