A group opposed to religious instruction in schools has filed a case with the Human Rights Review Tribunal over Christian favouritism in schools – but their battle may already be won.

The Secular Education Network considers religious instruction in schools to be favouring Christianity, and believes this is prohibited under the Bill of Rights.

SEN spokesperson Tanya Jacob told RNZ parents don't know what's being taught in classes and trust their children's school boards.

"There are cases where parents would be shocked. They'd be disgusted. Often they're just going along with it because they don't want their child to be singled out."

She says parents often think the classes are about a range of religions, when that is not necessarily the case.

"It's Christian indoctrination."

In Tauranga, though, religious instruction in schools appears to be on the decline. In the 2013 census, 56,526 people recorded at least one religious affiliation, while 46,920 recorded no religion. While population growth means the number of people affiliated with a religion has grown since 2001, so has the number of people with no religion – up from 26,412.

Tahatai Coast School principal Matt Skilton says they don't have religious classes at all.

'We're a values-based school, so we educate our children around things like perseverance, respect, responsibility and integrity, but it isn't focused on Christianity. We're very multicultural as well.”

Graeme Lind, principal of Tauranga's largest primary Greenpark School, says while they have had in the past, they no longer offer religious instruction due to lack of numbers interested.

'It's never been compulsory – parents opted into it. When we did have it, it was held during the lunchbreak, and it was only ever a small group that attended. Now we just have one show by Cool Bananas at the end of the year.”

For most schoolchildren at state schools in Tauranga, it would be Cool Bananas that exposes them to Christian stories and ideas.

The programme has been running since 1994, and currently covers 16 schools across the Western Bay of Plenty every week.

Cool Bananas coordinator Grant Vincent says they provide a values-based curriculum, taught from a Christian foundation.

'It's not there to push religion onto the kids – we just use Christianity as a base to teach from. We probably use a Bible story every second week, and use life stories of real historical figures or Grimm's fairy tales with a moral in the other weeks.

'We also have words to live by, which is generally a short verse from the Bible.”

He says many of the schools involved approached him to run the programme, which has become a substitute for the old Bible in Schools classes, in which each class would be visited by one teacher.

Those were increasingly hard to staff, so Cool Bananas now runs a schoolwide assembly once a week, with two or three facilitators.

Graeme stresses the programme is not about indoctrination.

'We're aware of the controversy that can be stirred up, so we're very careful not to push our opinions. Schools have to teach values as part of the curriculum, so we have a list of values we teach for them. We help to tick that box.”

In Tauranga, Bethlehem College and Aquinas College are two state-integrated schools with a Christian special character. It means while any student may attend, they must agree to embrace the school's Christian philosophy.

Bethlehem College Principal Eoin Crosbie says the Bible informs everything they do, including learning.

'In English, for example, you might look at some literature and compare their principles to those found in the Bible. We want our young people to see the world around them through a biblical lens.”

Bethlehem College was initially founded as an independent private school in 1988, but became state integrated in 1999 in order to become more accessible to Christian families struggling financially.