A serial offender sentenced to preventive detention after paying a 13-year-old boy for sex has had a second attempt to appeal against his conviction turned down.

Robert John Brown, 60, was convicted two years ago on four charges of sexual connection with a person under the age of 16 after meeting up with the boy, whom he met on the R18 dating app Grindr in 2018.

Brown has a criminal history stretching back to 1995, including 17 sex offences.

At the time of the latest offending, the boy was 13 but pretending to be 18 and Brown was 56, although he claimed he was in his forties.

After their first meeting online and exchanging photos, Brown sent the boy an image of four $50 notes.

Brown met up with the boy 10 times. On nine occasions he paid $200 and for the final meeting, he bought the boy a phone.

It was accepted during his 2020 trial in the Napier District Court that the sexual activity was consensual but judge ruled that Brown had not taken "reasonable steps" to make sure that the boy was over the age of 16.

Brown was sentenced to preventive detention, an indefinite jail sentence. Offenders can be released on parole but are subject to probation for the rest of their lives and can be recalled to prison at any time.

Brown was told he would serve a minimum term of five years and was placed on the child sex offender register.

He appealed against his conviction to the High Court, arguing that the boy told him he was 18, had been on an adult website, had claimed to be in tertiary study with a student loan, and had said he held a restricted driver's licence.

Despite all this, the appeal judge said that Brown had relied mainly on things the boy had said to him, without seeking to confirm the boy's age.

"It was sex for money and there was opportunity to clarify the situation – or just not have sex with him," the appeal judge, Justice Simon France, said in declining the appeal.

Brown then sought leave to appeal his conviction to the Court of Appeal, on the grounds that a miscarriage of justice had taken place.

The appeal bench of three judges found there were no errors in the lower court judgements giving rise to a miscarriage of justice, and turned down Brown's application.

Brown was first convicted of indecent assault on a boy aged 12 to 16 in 1995. In 2006, he was convicted on nine more charges involving three victims aged 13 to 15. In 2008, he was convicted on three more charges involving two victims – one aged 15 and one 22.

When he offended in 2018 he had been out of prison for three years and was subject to a 10-year extended supervision order, with a condition not to associate with anyone under the age of 16.