As kyzoo has stated, if you choose to specialize, the pay is anything but average. Also MBBS can take only 5 years plus internship, so it really isn't that long. You're done by he age of 24, and then you can choose to specialize and by age 30 you will be very well off financially.
Compare this to business/finance/law, which is the other major field where you have the potential to earn a lot of money in Australia. Most people who finish their degree are lucky to get employment for 40k per year when they leave Uni, and then it is just a matter of climbing up the corporate ladder. You won't be earning big money until you are 40+ years old. This is when it becomes very rewarding, as there is the potential to make huge amounts of money being a lawyer or board member of a corporation, but it takes a while to get there.
As far as engineering goes, that is another field with the potential to earn a lot of money, but jobs are very limited here in Australia. You would probably have to move to Europe if you wanted to be an engineer and earn amounts comparable to medicine and business/finance/law here in Aus.
It ultimately depends on what field you prefer and how long you are prepared to wait to see the fruits of your labour.
