WELLINGTON, Sept 7 - There are 10 operating now and two are due to open next year, so the four will bring the total to 16.

ACT leader David Seymour, who is under-secretary to the minister of education, announced the four new schools on Thursday.

One of them, City Senior School, will focus on science, engineering and maths.

The other three, Mr Seymour said, would follow the pattern of teaching used for students who were being "under-served" by the state system.

Charter schools are controversial and are fiercely opposed by teacherunions and the Labour Party, which has said it won't allow any more to open if it wins the election.

The rationale for them is that they can tailor their teaching for students who are failing or under-achieving in the state system.

They can set their own curriculum, don't have to hire registered teachers and can set their own term times.

The four new schools for 2019 are:

* City Senior School, central Auckland

* Vanguard Military School, Christchurch

* Waatea High, South Auckland

* Turanga Tangata Rite, Gisborne.

Charter schools - the government calls them partnership schools - are an ACT Party initiative and setting them up is part of ACT's support agreement with the government. 

The Post-Primary Teachers Association reacted swiftly to Seymour's announcement.

"The only thing charter schools do successfully is reward mediocrity by using scarce education money to prop up private owners," said PPTA president Jack Boyle.

"Opening charter schools is not going to raise the achievement of our children, it's not going to close any gaps, it's not going to level any playing fields."