It's like something out of a James Bond film — a helicopter landing on a boat, its crew leaping out to help at the 11th hour, as floodwaters surge around them.

Except this was no Hollywood blockbuster, rather, a real-life emergency on the Hawkesbury River.

Key points: The houseboat drifted 30km away from Lower Portland

The houseboat drifted 30km away from Lower Portland The Marine Rescue NSW crew described the incident as "amazing"

The Marine Rescue NSW crew described the incident as "amazing" The SES has made more than 260 flood rescues since Wednesday

As NSW's weekend of torrential rain sparked flooding in several areas, Marine Rescue NSW (MRNSW) was called to assist a houseboat drifting into the river at Gunderman about 85km north of Sydney.

When they arrived, they found a very large tree had lodged underneath the anchor, mooring and winch of a houseboat which had drifted about 30km from Lower Portland, with two people on board.

"They were literally stuck, they couldn't go anywhere and it was a very, very large tree," Sarah Adair from MRNSW's Hawkesbury branch said.

"The first thing we tried was to put a line around the stump of the tree and pull it off with our vessel but it actually snapped."

People work to free the boat from debris in the river. ( Supplied )

After several other failed attempts, the houseboat's skipper said a helicopter he owned was en route and could land on top of the boat, which was designed as a helipad.

"I was thinking 'Who is this? Is this Batman?'," Ms Adair said.

"It was quite amazing to see."

Shortly after, the chopper arrived with two people onboard and the six of them set about dislodging the tree from under the boat.

"We've never heard a houseboat with a helicopter on top of it before so it was an unusual experience for us."

The SES said the man was suffering from hypothermia. ( Supplied: NSW SES Bega Valley Units )

The downpour which started late last week and has spanned the entire NSW coast has caused widespread flooding.

This morning, a man was trapped by rising floodwaters and forced to cling to a tree for more than 10 hours on the South Coast.

The State Emergency Service (SES) said the man entered the Brogo River at Tarraganda, near Bega, about 6:00pm yesterday before being swept downstream.

The man grabbed hold of a tree and clung on for more than 10 hours until a passer-by spotted him at 4:00am today and alerted authorities.

A SES crew used a boat to rescue the man about an hour later.

The SES said the man was spotted about 4:00am. ( Supplied: NSW SES Bega Valley Units )

Local SES Commander Michelle De Friskbom said the man was cold but otherwise unharmed.

"He wasn't in great condition when we pulled him out," she said.

"He was suffering the effect of hypothermia ... even though it wasn't that cold.

"He had been in the water for quite a number of hours ... he is one very lucky man."

Meanwhile, people in Bega watched as floodwaters surged down their bushfire-ravaged town's parched river last night.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 49 seconds 49 s Residents in Bega rejoice as floodwaters surged down a dry riverbed. ( ABC News )

The nearby Brogo Dam is now full and spilling over — it was at just 10 per cent capacity a few days ago.

SES crews have responded to more than 260 flood rescues during the storms.

It has also responded to 500 calls for help for flooding and fallen trees overnight, bringing the total number of call-outs during the deluge to more than 13,000.

SES deputy commissioner Daniel Austin said thousands of people in the Hawkesbury and Nepean areas were still unable to return home due to flooding.

"As quickly as we can, and as quickly as that water abates, we will get people back in," he said.

"Their safety is absolutely paramount as is the safety of our crews."