The NSW Education Standards Authority said Wednesday morning's biology exam was interrupted after numerous schools received a threatening email.

They follow hoax emails containing bomb threats sent to about 20 NSW high schools on Tuesday, which led to thousands of Sydney HSC students having to abandon exams as schools were evacuated.

NSW education department secretary Mark Scott said those people making the threats may be deliberately creating "frustration and annoyance".

"We want students to be able to sit their exams...they've got a lot on their minds, it's a stressful time and it's been a stressful year - this is the last thing they need," he told ABC Radio on Wednesday.

"So this is particularly malicious for these students."

Mr Scott said despite the threats, more than 60 per cent of all HSC exams had already been sat and the whole process would be completed within a fortnight.

"We want to minimise disruption, prioritise safety and ... assure people we're working very closely with police to help swift identification of the people responsible," he said.

Earlier, Premier Gladys Berejiklian said Tuesday's threats were being investigated by the Cyber Crimes Squad.

"The students do not need to re-sit the exams, so we are working out the best way to make sure they're assessed," she told Sydney radio 2GB.

"It was a very scary thing, but I know police are on to it and I'm confident police will get to the bottom of it."

Education Minister Sarah Mitchell said the stunt was "a disgrace, not a funny prank" and warned severe penalties would be applied when the culprit was caught.

The main exams affected were legal studies and visual arts in the morning, but most afternoon exams went ahead as planned.

Ms Mitchell said any HSC student whose exam was disrupted by an evacuation would get special consideration when it was marked.

Australian Associated Press