A 49-year-old Dunedin man who spent nine years performing sex offences on two children in his care has been jailed for 18 years.

He was found guilty on 13 separate charges after a five-day trial.

The offending started around two years after the defendant began a relationship with the girls’ mother in 2004.

The victims were around eight years old when he started touching and massaging them.

This progressed to regular instances of rape, with one victim telling the court she was raped at least 50 times.

The offending took place in their home and the defendants workplace, while the younger victim was also extensively violated on a trip to a Bannockburn property with the defendant when she was nine years old.

The 49-year-old then threatened to commit suicide if either of the victims told anyone of the offending.

The court heard how the younger victim initially made a complaint to her mother in January 2015, only for her and her older sister - who would later testify against the man - to accuse her of lying.

Both sisters eventually complained to friends in May 2015 , and the matter was then investigated by police.

Crown prosecutor Marie Grills said this was offending at the highest scale, and sought a minimum 16-year jail term agreed to by defence counsel David More.

“The Crown would say the planning and premeditation was at a high level,” she said.

Judge Kevin Phillips told the defendant he needed to impose a term that would denounce his offending.

“The scale of offending is high, the breach of trust is high, the nature of the violation is high,” he said.

The man was sentenced to 18 years imprisonment, with a minimum non-parole period of 10 years.

Earlier, two of his victims gave emotional testimonies in court.

The elder victim told the court she over-exercised and stopped eating during the time of the offending, and weighed just 18 kilograms when she was 13 years old.

The younger victim started self-harming at 10, and spent some time in a mental health institute in Christchurch.

Judge Phillips said it would be difficult to imagine a higher breach of trust.

“You were their father figure. You had their mother around your finger,” he said.

“They were entirely and absolutely vulnerable.”