With predictions of a global population of eleven billion by the end of the century, Sir David Attenborough is concerned about his beloved natural world.

Australia is one of Sir David Attenborough's favourite places - particularly northern Queensland and the Great Barrier Reef.

"When people ask, what's the most exciting moment of all your wildlife watching career, I dont know what they think I'm going to say but what I actually say is, the first moment I actually dived.

"When you see a world of colour you never dreamed of, and you can move in and out of it in three dimensions... it's the most extraordinary thing!" he says.

So when he hears predictions of a population of eleven billion by the year 2100, he despairs.

"We all want places to put our houses and our gardens, and roads and airports. And all that space, where's it going to come from? Well it'll come from the natural world.

"And in the beginning we thought, 'Oh well there's plenty of natural world... well there aren't any anymore, we're using it up at an appalling rate," he says.

"There won't be room for anybody else, let alone the poor animals and plants... and we're dependent upon the natural world for the very air we breathe, but we would also be bereft of some of the greatest sources of delight and pleasure and beauty that we know of.

"So we've got a problem and we really have to solve it," he says.

David Attenborough will be in Sydney in November for his latest show The Third Dimension.