Health experts say young people are contracting sexually transmitted diseases such as chlamydia in epidemic proportions because they are not practising safe sex.

University of New South Wales Professor of sexual health Dr Basil Donovan says national figures show that last year more than 50,000 young people tested positive for the infection.

But he says only a small proportion of people at risk actually get tested, so the real figure is probably five times that.

"You may be looking at over a quarter of a million new infections each year," he said.

He says it affects both males and females, with implications for their future fertility if they are not treated.

Dr Donovan says there appears to be less and less adherence to safe sex among teenagers.

"One of the things about young people is they give most credence to what they hear from their friends and what they hear on television," he said.

"Right now chlamydia is a problem, because most people have no symptoms and no obvious signs, so they don't hear about it from their friends and they certainly don't hear about it on television.

"The current generation that are most at risk of chlamydia weren't even born when the Grim Reaper campaign happened."

He says health authorities must act on a public awareness campaign.