Divers collect 100kg of rubbish off city waterfront in half an hour on World Oceans Day.

Car batteries, coffee cups and condoms - welcome to the underwater junkyard that lies just off our city beaches.

People would be shocked, disgusted and embarrassed if they knew how much litter was going into their blue backyard each year, say divers who yesterday hosted a small clean-up dive off the Auckland waterfront to mark World Oceans Day.

In just half an hour, they hauled more than 100kg of rubbish up from the bottom of Okahu Bay.

They found a computer monitor, a smashed mirror, 41 glass bottles and jars, a dive mask, a pair of sunglasses and someone's underwear.

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Over the past few years, clean-ups made as part of Project AWARE's global Dive Against Debris effort have retrieved 8500 pieces of rubbish around New Zealand, most of it plastic bags and bottles, glass and paper.

Hundreds of pieces of litter have been recovered off the coast of Auckland, while nearly 4000 items were found off the Wellington coast.

But all of this was nothing compared to what was actually lying on the sea floor, said diver Andy Stewart, a member of the New Zealand Underwater Association who took part in yesterday's clean-up.

"They can't see it, so I don't think people are really aware of it," he said.

"But if they could be made aware of just what's down there, they'd think twice about chucking drink bottles over the side and make sure they rounded up their rubbish when they're at the beach."

Researchers estimate about eight million tonnes of plastic waste alone ends up in the ocean every year, including millions of disposable plastic bags.

An estimated 80 per cent of plastic rubbish in the ocean is carried out to sea by rivers, streams and the wind.

Countless marine animals, including whales, sea turtles and sea birds, choke to death after swallowing the rubbish.

Mr Stewart said it was also common to find assorted items such as appliances, fishing gear, furniture, tools and particularly foul litter like used condoms.

Kelly Tarlton's Sea Life Aquarium curator Andrew Christie, whose staff yesterday also dived to educate people about the effects of pollution on local marine animals, said that key messages concerning waste were taught to the tens of thousands of school pupils who visited the Tamaki Drive attraction each year.

"Being part of the solution is easy - if you are out and are not near a rubbish bin, take your litter home. Carry a bag with you and treat public environments like you would your own home."