A 21-year-old has been given a first strike warning after admitting having sex with an under-age victim after an internet hook-up.

Tony Jessie Nesbitt had already fronted up to the victim's father and apologised before his appearance in the Christchurch District Court to admit charges of meeting a young person after sexual grooming and having under-age sex.

Judge David Saunders remanded him on bail for sentencing on April 6, with a pre-sentence report on his suitability for home detention.

Defence counsel Andrew McKenzie said Nesbitt had strong family support and accepted that he had "let himself down very badly" and recognised the criminality of his actions.

He was actively seeking work after being stood down over Christmas from work at a temping agency. He was now living with his sister.

There was no suggestion of coercion in the offending.

"It was simply a situation that started through text messaging and Facebook and got out of hand," Mr McKenzie said. "He has taken the brave step of apologising to the victim's father."

Judge Saunders issued the three strikes warning and told Nesbitt that although he was getting a home detention report the offending was "in the range where imprisonment could be considered appropriate".