SYDNEY, Aug 17 - The breach is believed to involve three test papers being photographed on a smartphone at a school in the Ryde area.
HSC trial exams are provided to government and non-government schools in NSW and the ACT by the Catholic Secondary Schools Association.
Association spokesman Jim Hanna said the one class impacted by the breach would have to sit fresh exams in those subjects.
"While this is regrettable, it is necessary to uphold the integrity of the exam results," Hanna said in a statement.
It's the second serious HSC trial breach this month after physics practice papers were stolen from a cupboard at the prestigious Scots College in Sydney's east.
NSW Police is aware of both breaches and is investigating the Scots theft.