A six-year-old boy has died in hospital after being dragged limp from storm-swept seas on the NSW north coast.

The boy was swimming with family in shallow water at the unpatrolled South Ballina beach when his older brother got into trouble and his father swam to help.

They returned to find the younger boy floating face down in the water, with fisherman helping to pluck him from the surf.

Surf life savers and paramedics began CPR on the beach before the boy was flown by helicopter to Gold Coast Hospital in a critical condition.

He was unable to be revived and pronounced dead this afternoon, NSW police confirmed.

The near-drowning comes amid a severe weather warning for the NSW north coast as thunderstorms and heavy rains work their way south from Queensland, where five people were killed by floodwaters yesterday.