The jury deliberated for only a few hours before returning guilty verdicts to all 14 charges against the man, who has interim name suppression.
The verdicts came following a six-day trial which concluded yesterday.
Judge Kevin Phillips convicted the man of three charges of rape, seven of unlawful sexual connection and four of indecent assault.
The sexual offending started against both girls when they were young and continued until mid-2015.
The three counts of rape were representative charges - meaning they represented at least one occurrence of the offence - as were six counts of unlawful sexual connection and two of indecent assault.
The two victims embraced and cried after the guilty verdicts were read.
Judge Phillips said the man was subject to a three strikes warning and should expect a sentence of imprisonment.
``It will be a question of how long,'' he said.
The jurors' experience was ``harrowing'' and Judge Phillips thanked the seven women and five men for their service.
The offender was remanded in custody until sentencing in May.
