The monstrous east coast low unleashed a tsunami of waist-deep sea foam on the NSW south coast.

At Tathra, it banked up on the walls of the lifesaving club and cafe, and even spilled into the street on Monday.

One lucky puppy was even fished out by his owner after he went missing in the massive foam.

But experts have warned the sea foam could be dangerous.

"It's possible with big rain event sewage and other waste water can leak up and become part of it, " Dr Barry Noller, a toxicologist and University of Queensland Associate Professor, said.

"Because of all the run-off you don't know what's in it. There is a risk you could catch something."

He said while it "might be harmless and it might not be harmless", the only way to know is to test it.

The intense storm system that brought the sea foam battered the far south coast over the weekend.

The violent weather caused Tathra Wharf deck to slip off its pylons on Sunday night.

"There is a significant gap evident between the road pavement and the deck structure," Bega Valley Shire Council general manager Leanne Barnes said.

"Council staff and police have isolated the structure and it will remain closed until works are completed."

West of Eden, flood damage to the Towamba Bridge has caused it to be closed, pending assessment from engineers on Tuesday.

Further north, one boat was dragged under and another barge became wedged on a bridge on the Clyde River on Monday.

A pummelling king tide caused 50 metres of coastline to be eroded at Collaroy, with backyards and even a swimming pool toppling into the ocean.

- with the Bega District News, Batemans Bay Post and Eden Magnet