A group of Northern Beaches surfers have been hailed "heroes" after saving a man being swept out to sea during Sydney's wild weather.

Surfers were enjoying the surging floodwaters being released from Dee Why lagoon into Long Reef Beach on Monday, a common occurrence during periods of heavy rain.

Dramatic footage captured by a local drone surf photographer showed an elderly man being swept out from the lagoon entrance into the ocean.

READ MORE: How a Russian invasion could hit global economy and your wallet

"Everything happened super quick, we just got a wash out, we tried to keep his head up," Fabio Silvestre, one of the surfers, said.

Three surfers came to the man's aid but were no match for the wall of water and were sucked out of the lagoon.

One man on the side of the wall of water managed to offer the elderly man his hand and lift him to safety.

READ MORE: Australia will impose immediate sanctions on Russia

The three surfers who were sucked out also managed to pull themselves back to the safety of the sand dunes.

"He was in shock, he didn't say much, a bunch of guys asked him if he was okay, did he take in water and he was just shaking his head," Eugene Du Plessis, another surfer, said.

The surfers said the man didn't speak a word but attempted to go back into the water before giving up and heading to the carpark.

"I'm glad we were there, right time, and he went home," Mr Du Plessis said.

READ MORE: States bracing for more rain as two potential cyclones form

Conditions have eased at Dee Why Lagoon but it is still too deep to cross.

Locals said it is rarely as dangerous as it was on Monday during the city's wild weather.

"The water running out the way it did, I've never seen it like that," Mr Du Plessis said.