Among the 30,000 revellers at Splendour in the Grass music festival this weekend, about 1000 of them will have the sexually transmitted disease chlamydia – they just don't know it.

NSW Health authorities are hoping to test 3000 music lovers over the three-day festival and they predict about 100 will learn for the first time they are infected.

Splendour in the Grass 2018 kicked off on Friday and will run until Sunday. Credit:Splendour in the Grass

"They might not show the symptoms, so people think 'I’m okay', but without testing you don't know and later it may cause serious problems like infertility or pregnancy complications in women," NSW Health's Chris Bourne said.

“Music festivals present an opportunity to reach our target audience, raise awareness of STIs and reduce the stigma around testing.”