The offending occurred in Whakatane and he had previously been jailed for nine months for a similar offence in 2007.
In sentencing, the court was told how Shirley had been a friend of the girls' mother. She had decided to arrange a babysitter one night and he volunteered.
That day he drank 15 pre-mixed bourbon and colas, and not long after the mother left, violated the girls.
Shirley admitted the charges in June.
A victim-impact statement described how the damage that Shirley had caused "can't be measured". The girls had behavioural issues and didn't want their mother ever to leave the home, Justice Peter Woodhouse said.
Psychologist reports showed alcohol had been a "driving force" for Shirley's offending.
While acknowledging there was a risk, Justice Woodhouse deemed a lengthy minimum term of imprisonment as well as access to treatment would be appropriate punishment.
"You have never received controlled treatment for the alcohol abuse," Justice Woodhouse said.
"You have never received treatment to address the sexual offending ... You recognise the harm that you have caused. I regard that as particularly important."
Parole would also have the option of extended supervision on release and it was his first "three-strike" offence.
