Teachers are being left on temporary employment contracts for up to two decades, leading to a torrent of people leaving the profession for better pay and employment conditions.

NSW Teachers Federation president Angelo Gavrielatos told a committee examining insecure work that teaching had become less attractive to young people and high achievers.

"It hardly makes for a good recruitment strategy (when you say) 'Come on over, get a pay cut and be engaged in a temporary capacity'.

"(It's) fanciful"

The nation's most populous state would run out of teachers in five years time if the profession continues on its current trajectory, according to NSW education department briefing notes read to the committee by Mr Gavrielatos.

Three out of 10 teachers in the state are employed on a temporary or casual basis, and more than two-thirds of these people are early career teachers.

Young teachers are unable to "get a start on their life" because they cannot get loans and find it hard to establish themselves in the communities where they are temporarily placed, Mr Gavrielatos said.

The rate was even worse for TAFE teachers in NSW with 70 per cent of its 10,000 strong workforce employed on a casual or temporary basis.

There has been a 50 per cent increase in temporary contracts within the sector in the past seven years, the hearing was told.

Officials from education and manufacturing unions blasted the government's changes to industrial relations laws and the power it gave businesses to avoid offering permanent work to casual employees.

Employers are forced to offer a casual employee permanent full-time or part-time after 12 months of employment with some caveats in place, including them having worked a regular pattern of work for at least the last six months.

Businesses are able to avoid offering permanent work under some "reasonable grounds".

Mr Gavrielatos said TAFE NSW had used the clause to avoid offering a permanent role to any of its 7700 eligible casual staff.

The manufacturing union's national secretary Steve Murphy also took aim at the 12-month timeline, saying seasonal workers who work as much as 11 months a year forego this right.

Workers in the manufacturing sector previously had a right to transition to permanent employment after six months under their award. This was superseded by the new legislation.

"It is a significant step backwards for workers in the manufacturing industry," he told the hearing.

Australian Associated Press