A Whanganui teacher's sexual offending has been described by his lawyer as a "tremendous fall from grace".

The man — whose interim name suppression continues — earlier this year admitted charges of meeting a young person following sexual grooming and exposing a young person to indecent material.

He was sentenced to six months' home detention when he appeared in the Whanganui District Court yesterday.

The man was caught in an undercover police operation this year but the sentence also relates 2011 offending against girl who was 15 at the time.

They had earlier met on an online dating site and exchanged messages and photographs.

He later met and sat with the girl in a vehicle where Judge Josephine Bouchier said "there was touching going on but not of a particularly criminal sort".

When the girl turned 16 the man met her again at a hotel and performed oral sex on her.

The second lot of offending happened this year when an online profile of a 13-year-old girl was set up by an undercover police agent.

The man talked with the fictitious girl over a six-week period about masturbation, showed her photos of himself dressed in underwear and suggested meeting up.

The "girl" mentioned her age five times through the conversations and when it was mentioned she would be in Palmerston North the man suggested meeting up with her. This never occurred, however.

Crown lawyer Chris Wilkinson-Smith said the man had a high-risk profile because he was engaged in offending as recently as the middle of this year.

"This is not a case where it's happened in the 1980s or 1990s," he said.

"As recently as June this year he was trying to build relationships for a sexual purpose."

Mr Wilkinson-Smith argued the man's name should be added to the Child Sex Offender Register so he could be monitored post-sentence.

However, defence lawyer Roger Crowley said the man had lost everything.

"It's a tremendous fall from grace from a man who has led a blameless life," he said.

"He has lost his self-respect, his marriage, any contact with his children ... his career, his social status and his ability to function in the community properly because of this offending.

"It's not an excuse; it's simply where he finds himself ..."

Mr Crowley argued his client's actions since his arrest demonstrated he was no longer a risk.

"He engaged in counselling on day one; he was said to have real palpable remorse."

Judge Josephine Bouchier sentenced the man to six months' home detention and ordered him not to associate or contact the victim, anyone under the age of 16 or use any device which could connect to the internet.

She said a pre-sentence report assessed him as being "low risk of further sexual offending".

"I accept that his remorse is genuine and not situational, as in for himself," she said.

Judge Bouchier said it was not appropriate the man be put on the CSO Register.