Pemberton is 4 hours drive from Perth (343kms). From Bunbury it is around 156kms. From Perth you travel towards Bunbury, turning off onto the South Western Highway to Donnybrook, travelling through Balingup, Bridgetown and Manjimup and then 16kms further south to Pemberton.
Transwa operates a bus service several times a week from Perth to Pemberton. The journey takes around 8 hours. Buses depart Perth at around 12.20pm and arrive at Pemberton around 8.20pm. Of course there are numerous stops along the way such as Mandurah, Bunbury, Busselton, Margaret River and Augusta.
TransWA can get you to Pemberton. If this is your only mode of transport then it's a great facility, however the trip will take longer than if you drove yourself there. So it depends on how long you are going for as to whether you want to spend that amount of timie on a bus. An onscreen video is screened and there is a pit stop along the way.
The website is handy. You can find out all sorts of information about the trip and book your tickets online.