NSW Health says its biggest concern is the potential spread of chlamydia, which may not provoke symptoms but can cause infertility in women.

Almost half of the 29,000 cases of chlamydia in NSW last year occurred in the 15-24 age group.

STI expert Chris Bourne says gonorrhoea and syphilis are also a concern, but the teenagers should be most wary of chlamydia.

"Chlamydia has no obvious symptoms so people aren’t prompted to go for testing, but testing is important to find the infection without symptoms," he says.

"So people who don’t use condoms consistently should go see a GP and get tested, especially after having sex with somebody new."

He urged schoolies to always use condoms, which offer the "best protection".

"While schoolies should be a fun time after the stresses of exams, look after yourself and your mates," Dr Bourne says.

He directed parents and teenagers to NSW Health's "Play Safe" website, which provides STI prevention, testing and management help.

NSW Health is offering free STI testing services at select music festivals this summer, including the Festival Of The Sun in Wollongong in December.

"Getting tested is quick and easy, your results are completely confidential and chlamydia is easily cured with antibiotics," it says. "There’s no reason not to test."

It ran four successful pilots at smaller music festivals around NSW in the past year, which led to 71 people finding out they had a chlamydia infection and seeking immediate treatment.

STI rates in Australia are rising, with the number of chlamydia cases almost doubling over the past decade.

The department is spending $21.9 million this financial year in services to strengthen HIV and STI testing, treatment and prevention.

This includes $1.5 million in programs to engage young people aged 16-29 in good sexual health practices.