Violet McClintock's parents Lisa and Jeff are taking their daughter's school to court over its religious education programme.

New Zealand's largest religious instruction provider has won the right to join a landmark case brought against Bible in Schools programmes.

The Churches Education Commission (CEC) has joined the High Court battle between an Auckland father and his daughter's school over bible lessons.

Jeff McClintock, the father of a student from Red Beach School in Auckland, is taking on his daughter's school and the attorney-general in the High Court case.

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CEC applied to be added to the trend-setting case as an interested non-party during a hearing at the High Court at Auckland in May.

In a decision by Justice Susan Thomas, released on Tuesday, CEC was granted its wish to give evidence in the case.

Thomas said CEC's interests would be directly affected if McClintock was successful.

McClintock was bringing his case against Red Beach School Board of Trustees for allegedly breaching its duties under the Education Act 1989 and against the attorney-general for allegedly passing legislation that was inconsistent with the New Zealand Bill of Rights.

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McClintock's case did not originally include CEC as the religious instruction provider at his daughter's school was Life in Focus Trust.

But CEC said McClintock was implicitly challenging the rights of Boards of Trustees to exercise their discretion to allow voluntary religious education in state schools.

CEC provides religious education programmes to 667 state primary schools in New Zealand by 2582 Christian religious education teachers.

Justice Thomas said it would be "disingenuous" to suggest Boards of Trustees would not take notice if McClintock was successful in his declaration.

Justice Thomas referred to CEC as a religious education provider in her decision.

However, under the Education Act CEC was a religious instruction provider.

Religious education is part of a school's curriculum and taught during school hours by registered teachers.

Under the act, religious instruction is not part of the curriculum and is taught while the school is closed.

CEC spokeswoman Debra Hunt said the religious instruction provider was "really pleased" with the judgment.

There was no doubt CEC would be affected by the decision, Hunt said.

"We're just pleased to get to bring some accurate information to the court."

CEC's programmes had undergone some significant changes during the past 20 years, including the way it trained teachers and the material it taught.

Teachers were now trained to tell students the material was based on what Christians believed, rather than state beliefs as fact, Hunt said.

Increasingly, CEC was teaching Christian classes in schools that also had volunteers from other religious groups teaching students about their religion, Hunt said.

McClintock said he was disappointed at the High Court decision as it meant CEC would try and confuse the situation.

Secular Education Network (SEN) spokesman David Hines said CEC's inclusion meant the case would become more complicated and costly.

However, it was a good opportunity to have CEC's claims and teaching material assessed by experts and the court, Hines said.

McClintock is also seeking compensation for damages from Red Beach School but Justice Thomas said CEC was unable to seek costs as part of the case.

Meanwhile, SEN had been in mediation with the Ministry of Education through the Human Rights Commission.

* This story has been updated to clarify the difference between a religious education provider and a religious instruction provider