Hi there, we've came over from England 8 weeks ago. We came over on a 489 visa. We have been living with friends just above Perth but couldn't find any work regionally there for my husbands occupation. We checked Mandurah out and spent a week at Bunbury too. We couldn't find any work at Mandurah and didn't think there was as much to offer as there was at Bunbury. We have two young children and felt that Bunbury offered a lot more workwise and a good all rounder in general. We have got a rental at Treendale, Australind. The children go to school there too. It has a village feel to it which is what we are used to and think its great to have Bunbury about ten mins away where you can get anything you want from there too.
I would seriously check them out and find what works best for you and your occupations. We have found that we totally rely on sites like Seek, jobrapido, wajobs to look for work. I would check the work sites out and go with how you feel as to how much comes up for the area you are qualified in. Mandurah is a lovely place and I know you have a train route to Perth but if you have a car it only takes an hour and a half to Perth too so I wouldn't rule out Bunbury.
We felt that Bunbury had more to offer than Mandurah personally although they are both lovely places. Mandurah seemed much smaller and couldn't see much to do there even for the day. Being at Bunbury you also have some beautiful places to visit like Bussleton and Dunsborough, Margaret river too.
Good Luck!