A West Australian man who used a mask and a voice-changer to disguise himself before sexually assaulting his stepdaughter has been sentenced to nine years behind bars.

The man was taking the 18-year-old woman for a driving lesson near Collie in the South West last December when he staged a break down and went to call for help.

He returned a short time later in disguise, tied the teenager to the car seat and sexually assaulted her.

Two months later — while on bail for the previous offences — the then 37-year-old disguised his car as a police vehicle by using blue and red plastic light covers and wiring the headlights to make them flash.

He stopped the woman as she was driving her car in Bunbury before again putting on a Kylo Ren mask from the Star Wars movies, forcing her to strip naked and taking photos of her.

The District Court in Bunbury heard the woman had been planning on moving in with her boyfriend and the man had hoped the first attack would scare her off the idea.

He told police the second offence was carried out in the hope the original charges against him would be dropped.

Sentencing the man, Judge Felicity Davis described the assaults as planned and premeditated.

"It did not even cross her mind that it was someone she knew. She loved and trusted you," Judge Davis said.

The man will be eligible for parole after serving seven years of his sentence.