Church recruiters are using schools as "mission fields" to convert children to Christianity, despite claiming their aim was to educate not evangelise.

The country's largest provider of religious instruction, the Churches Education Commission, told its followers in a recent newsletter that schools were an "under-utilised mission field".

"Churches by and large have not woken up to the fact that this is a mission field on our doorstep. The children are right there and we don't have to supply buildings, seating, lighting or heating," commission director David Mulholland wrote.

Christian followers were also encouraged to join school boards so they could have "more influence" on holding religious study in class.

Public schools are secular but can choose to "close" in the middle of the day for religious lessons.

Rationalist David Hines said the loophole allowing Christian education undermined the secular education system.

"The teachers are all evangelical Christians ... the values do not include respect for other religions or for secular value systems."

A Sunday Star-Times report last week that a group of rationalists were campaigning to ban religion from public schools prompted a flood of letters from readers, both for and against.

One reader, Liz Donnelly, said the moral state of the nation was a reason to teach basic Christian principles. "Looking at New Zealand at present, we see the lawlessness, lack of respect to those in authority and high teenage pregnancies."

However, other parents argued that good values were not exclusive to Christian teachings.

"You don't need to use religion to teach basic values. They could be incorporated into normal class time," Nathan Grange said.

Parents also told the Sunday Star-Times about religious groups using a preacher in school halls.

The commission teaches in 800 schools nationwide. Last week director Simon Greening said Christian teachers were in schools to educate, not evangelise.

His colleague Mulholland, who referred to schools as mission fields, said the organisation's aim was to "sow a seed" of Christianity, not to convert children.

Mulholland accepted that parents might misinterpret the word "mission" as conversion, but the newsletter involved was aimed at Christians who would know the difference.

"It's helping people be aware of Christianity, what the Bible says, what Christians believe about the Bible. It's not our responsibility to convert."

Most children do not attend Sunday schools and their parents are often "ignorant of Christian things", he said.

"This is an opportunity for that seed sowing to take place."

Individual boards of trustees decide whether to allow Christian education in their schools.

Ian Leckie, president of teachers' union the NZEI, said there was no need to review the rules of religious instruction in public schools, as the system was working.

"I dont think anybody intends this as a [religious] conversion; it's more meeting a social need that's being asked for by the community."

Changing to a compulsory "opt-out" system would also create an unnecessary administration burden.

In 2006, the Government back-tracked on a proposal to move to an "opt-in" provision for religious education following a public outcry.

An Education Ministry spokesman said there were no plans to amend the legislation.

Members of the Keep Religion out of School group estimated that about 40 per cent of public schools closed for religious instruction. A school may close for up to one hour a week, up to 20 hours a year, for religious instruction. Parents can withdraw their child from the class, and the child would normally sit in another class or the library.