A feral deer has been euthanased after it was washed into the ocean off a New South Wales beach.

Key points: A deer has had to be euthanased after a pair of the animals were swept into the ocean at a Wollongong beach

A deer has had to be euthanased after a pair of the animals were swept into the ocean at a Wollongong beach Lifeguards with jet skis were deployed to herd them back to shore

Lifeguards with jet skis were deployed to herd them back to shore Council has been concerned about the feral pests for some time, but the rescue is thought to be a first

Onlookers reported seeing two deer standing on rocks at a beach in Wollongong being lashed by strong waves before falling into the water.

Lifeguards helped to rescue the pair but Wollongong Council's acting Mayor David Brown said one had to be euthanased out of concerns for its welfare and public safety.

"One of them got stranded behind [local ocean pools] the Continental pools and unfortunately it had to be tranquilised and euthanased," Councillor Brown said.

"Having a wild animal that close to traffic and people was a real safety issue so it had to be put down.

"It's a large animal, it's frightened and likely to charge off anywhere and could easily trample somebody walking or cycling in that area."

The other deer is believed to have returned to bushland.

'Lost, confused and scared'

Local man Rick Miller said it was an odd sight.

"There were a couple of big deer that got on to the beach and on the rocks, and they were pretty petrified and a bit of a crowd gathered to watch them and called emergency services," Mr Miller said.

"Everyone was just watching these deer sitting there heading out to New Zealand, and people were speculating — 'how well can deer swim?'"

Locals believe the animals had wandered down to the beach from the Puckey's Estate Nature Reserve.

NSW Fire and Rescue and NSW Police responded and had crews on the beach attempting to help.

Courtney Steltenpoon was having a coffee at a beachside cafe and saw the scene unfold.

The deer were stranded on a rock before they were swept into the sea. ( Supplied: Jen McDowens )

"I walked down to the beach and there were lots of people pointing and at first I thought it was a whale," Ms Steltenpool said.

"I looked a little bit closer and I saw the two little deer on the rocks, and the surf was quite big and they looked pretty lost, confused and scared.

"The second I realised they were deer and where they were I was quite concerned.

"I was worried they were going to fall in the water and drown, but we consulted Google and apparently deer are quite strong swimmers when they need to be, so that settled my mind a bit."

Lifeguards in jet ski rescue

The animals swam further out to sea as people tried to help.

Wollongong Council lifeguards used jet skis to steer the animals back to the shore.

For nearly a decade Wollongong City Council has been raising concerns about feral deer problems.

This is believed to be the first time lifeguards have had to rescue deer.