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Police were called to the scene about 5pm after the 48-year-old man and his 12-year-old son became stranded by the rising tide while fishing on a northern point near the mouth of the river.
Constable Nathan Atkins, of Waimate, said the man and his son did the right thing by not attempting to swim for safety.
"He probably made the right decision, I think. They are more embarrassed really, as he couldn't swim and he's decided not to risk it.''
The man instead contacted his wife, who called emergency services.
The pair were airlifted to safety about 5.30pm by a rescue helicopter.
A boat, owned by a Glenavy fisherman, was also placed on standby to assist.
The man and boy were not injured.
An Oamaru woman at the scene, who asked not be named, had seen similar incidents in the past and said people needed to take more care when fishing or boating in the area.
"I actually think they should be very mindful of the tides.
"There are quite a lot who go out there and don't know what the tides do. They don't know where it's safe.
"They go out there at low tide and don't realise the water is coming in and get stranded.
"I've seen a lot with no life jackets, not bringing appropriate oars, losing oars and very limited boat skills. Since Christmas ... I am aware of half-a-dozen instances where skilled boaties have helped people out.''