A shortage of maths and science teachers in NSW public schools will see the state government employ 900 extra full-time teachers for the Sydney suburbs under most pressure from population growth.

The new teachers would be for areas such as north-west and south-west Sydney, the government says.

A lack of STEM-qualified teachers in high schools and anxiety about teaching STEM subjects among primary teachers has been blamed as a reason for a fall in enrolments in high-level maths and science subjects.

NSW Premier Gladys Berejiklian is seen with students from the Taverners Hill Infants School during a visit earlier this year. Credit:AAP

It is also seen as contributing to Australia's overall declining performance in international tests.