Tourists visiting Australia's natural wonders in large numbers are threatening the environment they want to enjoy, a leading futurist says.

Sydney-based writer and futurist Richard Watson warned an eco-tourism and green travel conference in Adelaide on Monday that there was a dark side to tourism. "In the short term there is a definite problem that people are possibly going to destroy the very thing they came to experience.

"That is without a shadow of a doubt true depending on what the economy does and there is going to be a very, very significant growth in population numbers over the next 10, 20, 50 years."

However, Mr Watson said in the long-term oil prices could hurt tourism badly.

"When oil starts hitting $200 and $300 a barrel - and it's not if it's going to hit, it will hit eventually - that is going to fundamentally change all this.