The nine meter "Pirate King" vessel was washed out of the Shoalhaven River on Monday with floodwater, and eventually drifted north overnight to be found near Bass Point.

"When I spoke to the owner they couldn't believe it," said a Marine Rescue spokesman.

"Somehow it managed to drift all that way without hitting anything."

The owner has been contacted and will survey the damage to the catamaran on Monday.

Authorities expect the vessel to eventually be sailed back home, as it was in "fairly good condition" considering it's escapade.

9:05am: Marine Rescue Port Kembla is going to shadow the Shellharbour rescue towing the catamaran in to shore at Port Kembla Harbour.

Spokeswoman Kate Woods said the catamaran may have lost its rudders and the extra crew will be able to assist in manoeuvring the large boat on to a mooring.

8:55am: A nine meter catamaran is currently under tow by Marine Rescue, after rising floodwater on the Shoalhaven River swept it out to sea on Monday.

Kate Woods from Marine Rescue said the vessel, named "Pirate King", was washed down the river before sitting at Shoalhaven Heads.

At some point on Monday someone attempted to throw an anchor over the vessel but it didn't hold.

"It sat at the entrance throughout the day, but the anchor hasn't held it overnight," she said.

Overnight it drifted north and was found about four nautical miles off Bass Point, near Shellharbour on Monday morning.

Marine Rescue are towing it to Port Kembla Harbour.

EARLIER: A rescue operation is underway off Shellharbour on Tuesday morning, after a catamaran was swept out to sea.

A crew from Marine Rescue Shellharbour has arrived on scene to retrieve the nine meter vessel, which was washed out to sea by floodwaters on the Shoalhaven River.

It's understood the catamaran broke its mooring on the flooded river sometime on Monday, and was swept through the entrance at Shoalhaven Heads cut open by floodwaters, drifting north about 17 nautical miles overnight.

A Marine Rescue spokesman said the boat, named "Pirate King", had drifted about four nautical miles off Bass Point and at 8:30am Monday was under tow into to Port Kembla.

"It broke its moorings in the Shoalhaven somewhere," he said. "We have been tracking it on our system and we've currently got it in tow."

It's unknown at this stage if anyone was aboard the drifting boat.

Heavy rain over the weekend saw the Shoalhaven River break its banks in multiple locations, with a peak at Nowra and Terara not seen since 1991.

August has been unusually wet for the Illawarra and South Coast, with rainfall totals from the last four days nearly five times the monthly average in some areas.

From Friday to Monday, Nowra recorded a total rainfall of 373mm compared to the monthly average of 76mm.

While Kiama recorded around 106mm of rain from Friday to Monday, compared to the monthly average of 80mm; and Albion Park recorded about 230mm compared to the mean monthly rainfall of 53mm.

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