With the current rate of the MYR and the AUD, it will be a very expensive thing to rent a car.
Car rental companies also put a hold on the value of your credit card, thats money you can't use.
Plus insurance charges.
Toll road charges also have to factored into the cost of the rental.
Car rental companies also ask from which country you come from, and can charge higher on that, there are many car rental companies at MEL airport Tullamarine.
If I were you, I would fly one way, its cheaper to fly, and then drive back to MEL.
Myr3000 according to xe.com will only buy you Aud982.
If you want to see Aust, and its your first time, go with Greyhound which have a km pass, or Firefly Express.
I would let someone do the driving, get used to it, and then, next time, drive.