Police are searching for man in the Tweed River after his five-year-old boy 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.

He was pulled from the river and given CPR by a man believed to be either an off-duty nurse or paramedic, the spokesman said.

The boy was then treated by paramedics on the boat ramp before being transferred to Tweed Heads District Hospital, and later 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.

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

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

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

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