A repeat child sex offender who sent explicit messages to a police officer posing as a 13-year-old girl and been jailed for 18 months.

Details of how Graeme Murray Purvis, 57, who has a long criminal history of child sex offending, was caught have been revealed at Christchurch District Court where he was sentenced today.

Purvis set up an account with online social network Chatiw in June this year and contacted a 13-year-old girl, who was actually a detective with the Online Child Exploitation Across New Zealand Team.

Using Google Hangouts, Purvis then sent the undercover officer a link where they exchanged more than 500 messages, many of them explicit.

Purvis said he would be visiting Christchurch in late July and suggested the "girl" tell her caregiver she was having a sleepover with a school friend so that she could spend the night with him.

On the morning of the arranged meeting, he sent more sexually-explicit messages - including a photo and video - and tried to convince her to visit his place.

They ended up meeting at a frozen yoghurt shop in Riccarton where Purvis was arrested.

He ended up pleading guilty to charges of exposing a young person to indecent material, travelling to meet a young person after sexual grooming, and failing to comply with reporting obligations as a registered child sex offender.

Today, defence counsel Joshua Grainger said Purvis wants to undergo treatment but he has been unsuitable because his risk level is so high.

Judge Stephen O'Driscoll highlighted a pre-sentence report which said Purvis showed "little concern" for his offending and held "heavily-entrenched beliefs in his attitudes towards authority".

The judge told Purvis he hoped that he would do something to turn around his pattern of offending.

He was sentenced to 18 months' imprisonment with six months of standard and special release conditions, including a ban on using any electronic device which can access the internet without authorisation by probation officer and not to associate or contact anyone under 16.

In 2016, Purvis was jailed for nearly three years after taking his brother's laptop and downloading child pornography – just days after a sentence for similar offending ended.

And in 2009 in Dunedin, he was jailed for objectionable publications and trying to sexually groom a 15-year-old girl.