Extension science units could be offered for the first time in the HSC under a radical new proposal from the NSW Board of Studies.

The board's draft writing briefs, released on Monday, have revealed that three-unit chemistry and three-unit physics could be a part of the state's curriculum by 2017.

There is also an option for a super-science extension subject with elements of physics, chemistry and biology that could be combined together to test the state's most talented students.

The proposal was revealed as 17,500 students sat the first of the science exams, biology, on Monday.