A school principal who asked parents if their kids didn't want to sing the national anthem for "cultural or religious reasons" says none have asked to have their kids opted out. 

Carlton Primary in Whanganui sent a text to parents this week asking if they could get in touch "if you do not want your child to sing the national anthem at assembly due to cultural or religious reason", NZME reports.

"I was standing looking out at the assembly, the mass of children standing there, and realising when you looked along the lines of children - jeez, there's quite a lot of kids not singing," principal Gaye O'Connor told NZME.

"Is that because they can't be bothered singing today? Is it because they don't know it anymore and we need to re-teach it again?"

She said just like some families don't celebrate Christmas or birthdays, some might not want their kids singing the anthem, titled 'God Defend New Zealand'. 

One grandparent told NZME the text showed a lack of respect, saying they weren't the only unhappy caregiver.

But O'Connor said since the text was sent on Wednesday, she's not heard any objections.

The New Zealand Principals Federation backed Carlton Primary's proactive measure, saying parents "parents have the right to indicate they do not wish their child to be participating" in singing an anthem that contains references to God.

The Ministry of Education said it was up to schools to decide whether or not to sing the anthem, and the school had "demonstrated good practice" in asking parents.

Written in the 1800s, 'God Defend New Zealand' didn't become an official anthem until 1977 - a position it shares with 'God Save the Queen'.