A 55-year-old man who sexually penetrated and indecently dealt with girls as young as five in WA's south in the 1990s has been sentenced to five years behind bars.

Neville John Cotter, who is already in protective custody at Perth's maximum- security Casuarina Prison for other child sex offences, was a family friend when he abused the girls in the Esperance region between 1993 and 1998.

District Court of WA Judge Laurie Levy said Cotter, who will also face Esperance Magistrates Court next month over sex offences against adults, had "groomed the families" to get close to the children.

AAP