A father and son's New Year's Eve fishing trip almost ended in tragedy after their boat was swamped by high seas and they spent about one hour in the water before being rescued.

The pair were in Golden Bay on the northern tip of the South Island this morning when their boat was swamped by waves, police said.

The 65-year-old father was forced to ring his wife using his cellphone after his marine radio failed to work, before he and his 33-year-old son spent nearly an hour in the sea wearing lifejackets before they were picked up by local volunteers.

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Nelson Marlborough Rescue Helicopter then took the father to hospital with mild hypothermia.

"This was another great rescue by our local volunteers and a good reminder to check your safety equipment before heading out," Senior Constable Jonathan Davies said.

Nelson Marlborough Rescue Helicopter's duty pilot said the father and son had already been picked up by another boat by the time his team arrived overhead.

The pair's "dinghy", which had earlier been overturned, had also been recovered and tied to a buoy.

The helicopter then met the boat carrying the father and son at the Collingwood boat ramp, where a medic assessed the pair.

The helicopter pilot said he didn't get the chance to talk very much to the father during the trip to the hospital, but said the whole team was relieved at the outcome.

"It could have been far more serious and they were probably lucky there were a few other vessels and people fishing in the vicinity," he said.

Meanwhile, two people have been rescued from waters off North Otago this evening after their boat sank.

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A police spokeswoman said police and fire crews were called at 5.05pm today to reports of a vessel having gone down off Glenavy, north of Oamaru.

Two people were believed to be in the water, she said.

An Otago Regional Rescue Helicopter was dispatched and the Coastguard called.

The spokeswoman said later the pair had been rescued and as of 6.40pm were en route to Oamaru Hospital, both in a moderate condition.

It appeared they had been rescued via the helicopter's winch.

A call came over maritime radio channel 16 (the distress channel) about 5.45pm, describing the pair as 400m offshore from Fisheries Rd, Glenavy, at the mouth of the Waitaki River.