A man is in a critical condition and a woman is also injured after falling down a cliff in Sydney's north while attending a 21st birthday party.

Jordan Bowling and Emma Korell fell about 15-20m down a cliff near the party in Sydney's north on Saturday night.

The accident happened on Quarter Session Road at the edge of the Berowra Valley National Park at Westleigh, and sparked an hours-long rescue effort.

READ MORE: NSW Ambulance service overwhelmed four times in the past fortnight

The man whose birthday it was had gone looking for the pair after they wandered off from the small party.

He said Mr Bowling slipped first and Ms Korell fell trying to save him.

The pair was eventually winched to safety after specialist operations paramedics and Fire and Rescue NSW were called

Mr Bowling suffered head injuries and Ms Korell had leg fractures.

Inspector Charnan Kurth from NSW Ambulance described the area as dangerous

"Fortunately there was no rain last night but it has been very wet and the rough and very steep terrain is very dangerous as well for our paramedics," he said.

Neighbour Vicky Jacquet said it was a steep area.

"It's a bit sketchy getting down there. It can be very steep initially getting to the rock," she said.

The pair was rushed to Royal North Shore Hospital.