A coach is a really good idea for interview preparation.
In fact, one of the problems that often cause candidates to seek out coaching is that in past attempts they have relied on other types of people for feedback, such as family and colleagues.
Often this type of feedback can be misleading and worsen your performance.
There are lots to consider in choosing a good coach. Price is obviously one. But like in most situations you generally get the quality that you pay for. So don't just choose the cheapest coach.
Look for coaches who can provide good testimonials and references from past clients.
Look for coaches who have had actual experience in coaching others for the type of interview you are facing.
Coaches who offer a free or discounted initial session are often a good choice as they wouldn't generally do this if they were not confident in the service they provide.
Finally, don't just choose a coach in your local area. There are actually not that many really good interview coaches in a place like Australia.
You are far more likely to find a good coach by looking for online coaching options.
Online coaching has a number of benefits over attending a live session. For one it eliminates travel. Secondly, often you can get more convenient booking times. And finally, you generally can also get a record of your session to review later.