Diver trying to break world record with 24-hour underwater clean-up in Sydney Harbour

A world record attempt is underway with a diver trying to collect as much rubbish as possible from under Sydney Harbour.

Dean Cropp is trying to break the record for a 24-hour underwater clean-up.

"What you see on the surface is sometimes only 25-30 per cent," Mr Cropp told 9News.

"A lot of it sinks and it's down there, out of mind out of sight. And no one's there to clean it up."

Mr Cropp, an underwater photographer, is scuba driving for 24-hours straight to retrieve single-use plastic from the ocean floor.

He will be battling not just exhaustion but some underwater creatures.

"I'm not so worried about sharks," he said

"There's actually a tiny little organism that I have to watch out for which is super dangerous. It's called a blue ringed octopus."

Once Mr Cropp entered the water at Mrs Macquarie's Chair he has to stay underwater for 24 hours.

There are 20 air cylinders at hand for him and a paramedic on standby. Visibility is around three metres.

The rubbish being found over the coming hours will be taken out and sorted.

It's hoped most, if not all of it, will be recycled .

Some of it will be turned into permanent use plastic bottles for zero-waste startup Zero Co - the company behind the dive.

"I think it's important for us to showcase how much plastic is going into our waterways, is going into our harbours, is going into the ocean but also the solution that we've come up with to solve this single use plastic problem," founder of Zero Co Mike Smith said.