Police are searching for a man in the Tweed River, on NSW's north coast, after his five-year-old son, whose heart had stopped, was fished out of the river by a passerby yesterday.

The boy was discovered floating face down in the river about 100 metres from a boat ramp, off Chinderah Bay Drive, a short time before 3pm, a police spokesman said.

According to initial reports, the boy had no pulse and was not breathing after being pulled from the water, an ambulance spokeswoman said.

However, his heart started beating again after CPR given by the passer-by, who is believed to be either an off-duty nurse or paramedic.

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The boy was then treated by paramedics on the boat ramp before being transferred to Tweed Heads District Hospital. A tube had to be inserted down the unconscious boy's throat to help him breathe and he was later transferred to Brisbane Hospital in a critical condition.

At the time, police had been unable to identify the child and no one had been reported missing, said Inspector Carey, Crime Manager of Tweed/Byron local area command.

About 8.20am, water police found a boat submerged in the Tweed River "almost directly opposite'' the spot where the boy was discovered, Inspector Carey said.

He could not say how far from the boat ramp it was found, or what sort of boat it was.

Police have towed the boat to a neaby island and were investigating whether it belonged to the father.

As the search for the missing man continued, police this morning were in regular contact with the boy's mother, who was currently with her son in hospital, said Inspector Carey.

Police later found a car and boat trailer in the carpark, and discovered the car belonged to the 35-year-old father of the rescued boy.

"There are witnesses who saw them fishing in a small dinghy about 1pm," said Inspector Carey.

"Apparently they are known to regularly fish in that spot, there's a bit of history there," the police spokesman said.

The man's car was a white Ford Falcon station wagon, with Queensland registration 178 KEA.

"I'm not sure where they're from, but the boundary with Queensland is just 10 kilometres away," Inspector Carey said.

Police have been searching for the man since 5.30am this morning.

"It's a large river, a wide river and there's been at least two tide movements since the time the child was first located and we're heading into a third, so they have an extensive area to search," Inspector Carey said.

He said no other items had been discovered by search teams.

The search involved the Volunteer Marine Rescue, NSW State Emergency Service, Coffs Harbour water police and the Westpac Rescue Helicopter, he said.

Conditions for the search were calm, Inspector Carey said.

Arthur Seers, who owns a newsagent on Chinderah Bay Drive overlooking the Tweed River, said weather conditions on the water yesterday had been the best since winter started.

"It was beautiful, like a pond,'' he said.

There were a number of businesses alongside the boat ramp where the boy was found, including two caravan parks, a takeaway food store, a Chinese restaurant, a cafe and his newsagency, he said.

The boat ramp was frequently used by recreational fishermen to launch their boats, he said.

"There's always people there, especially on a day like yesterday.''

However he said many people he had spoken to had not seen the pair yesterday.

Anyone who may have seen the white Ford Falcon or a car matching that description was asked to called Crime Stoppers on 1800 333 000.