You can drive to Mt Barker via the Albany Highway which travels from Perth (359km) in the north, or from Albany (50kms) to the south. The Muirs Hwy comes in from the south east (Denmark 53kms & Manjimup 156km). Albany is nearest biggest town around 50kms to the south.
Transwa run buses from Albany to Mt Barker which takes around 40 mins. There is also a bus which runs daily from Perth, taking around 5 hours.