A hiker doing the popular Otter Trail in the Tsitsikamma National Park was swept out to sea and is presumed to have drowned while crossing the stormy mouth of the Bloukrans River on Wednesday.

A seven-hour air and sea search for Marthinus Esmeyer, 44, of George, was called off late on Thursday.

Plettenberg Bay's National Sea Rescue Institute (NSRI) chief Raymond Farnham said: "No one can survive these sea temperatures and conditions.

"The sea is very stormy and the waves are still very big and battering against the rocks.

"We've covered many hundreds of square miles and found nothing. Mr Esmeyer is presumed to have drowned."

Esmeyer was hiking with his teenage son, also Marthinus.

The trail crosses the Bloukrans River at the mouth and can be dangerous if the river is flowing fast and the sea rough.

Because of stormy seas and the recent heavy rainfall in the Bloukrans catchment area, SA National Parks staff warned the hikers not to cross the river, but rather to take the "escape route", an optional trail which would take them inland to cross the river at a bridge.

Some hikers took the alternative route, but Esmeyer, his son and two young German women opted to cross the river.

"I was told they were in water about waist-deep. The German girls' rucksacks were washed away and Esmeyer was trying to help them. He apparently lost his footing twice, and was then knocked over and washed out to sea.

"The river mouth is fairly narrow and then widens upstream. The seawater comes in, builds up in this wider part and then suddenly pushes out in a helluva strong backwash. He was washed out to sea and eastwards," Farnham said.

One of the hikers raised the alarm on a cellphone.

National Parks staff immediately alerted the NSRI at Plettenberg Bay.

Farnham said he called in the assistance of the air force which dispatched an Oryx helicopter, which had been at the Wilderness, to scan the sea.

The search was called off at dusk on Wednesday and resumed at first light on Thursday.

Esmeyer's wife and his two daughters travelled from George to the park, where they have been staying during the search.

Land parties will continue the search on Friday.