Freediver's daredevil attempt

FREEDIVERS will literally be holding their breath on Sunday as a daredevil world first is attempted off the northern NSW coast.

Mike Wells, 39, a 19-year veteran of freediving - the art of diving without oxygen tanks - will swim through Fish Rock Cave, said to be Australia's longest ocean cave, on a single breath.

Nobody has ever completed a freedive through the 120-metre underwater structure.

On top of the risk of swimming through such a confined space for around three minutes, the water around Fish Rock Cave, two kilometres off the South West Rocks coast, is infested with sharks.

"Apart from being in a tunnel, sharks are the biggest risk, for sure," Mr Wells told AAP.

"Swimming through there, it's going to be pitch black and you just never know what could leap out at you."

Mr Wells is undertaking the challenging dive to raise money for the Australian Marine Conservation Society (AMCS).

The organisation carries out important work in the area, including looking after its large population of grey nurse sharks.

The creatures are often found with fish hooks in the mouths and ropes around their tails, according to Mr Wells' close friend and freediving champion, Sacha Dench.

She will be on hand along with other divers to assist on Sunday.

"There are risks of course, but all the homework has been done. We've assessed everything.

"The area around the tunnel really needs looking after, it is an amazing place and hopefully this will raise some money for the AMCS to carry on that work," she said.

To complete the epic dive Mr Wells will first have to swim down 26 metres before entering the tunnel - in some parts only a metre wide - and swimming along its entire 120-metre length.

Once at the end he faces a swim of 14 metres to the surface.

"It should take about three minutes, hopefully," Mr Wells said.

"I've done longer dives before but nothing like this in terms of negotiating small spaces.

"I've dived down there before and I actually get to recognise some of the sharks, I recognise their markings and know them individually.

"Hopefully they'll recognise me and let me get on with it."

To donate to the AMCS fund being collected in honour of Mr Wells' dive visit www.amcs.org.au.