It is very important to consider the opportunity cost of anything we do. Opportunity cost represents the benefits an individual misses out on when choosing one alternative over another. For example, the benefit you miss out on by choosing a 6 year medical program vs. a shorter 5 year program. Similarly, you should consider the benefit you miss out on by going to graduate medical program (which takes a minimum of 7 years) instead of going to a five year school leaver entry program. Keep in mind that the prestige of the university matters very little when it comes to medical degrees.
Assuming a conservative figure of $250,000 per year as the average earnings of a doctor over their lifetime, the opportunity cost of going to Adelaide or UNSW (6 year programs) rather than Monash or WSU, would be that amount. So the question you need to ask yourself is: If you had to pay $250,000 to go to UNSW or Adelaide, would you do it? If you would, then go ahead. If not, but you still want to attend UNSW or Adelaide, then it is not a rational decision.
Similarly, the opportunity cost of going to USyd, Melb Uni (or other graduate entry programs) rather than Monash or Newcastle (which takes 2 years less) would be half a million dollars. So the question to ask is: If you had to pay $500,000 to go to USyd/Melb Uni would you do it? If you would, then its fine. If not, but you still want to go to USyd or Melb Uni, then it is not a rational decision - it is an emotional one.
Furthermore, the above examples assume that you will definitely get into a graduate medical program after you complete your first degree, which is often not the case.
Finally, getting into specialties is getting more competitive, so a one or two year headstart by completing a five year undergraduate program will give you a massive advantage.