Hey Ry,
It's best to fly into Canberra, it's only 2 and a half hours by hire car, or, you can get a connecting flight into Merimbula itself, catch a taxi to the "Tween Waters Cara Park" and providing they don't go to Tathra or Pambula, then you won't need wheels at all or just bum a lift.
Sydney and Melbourne are about equidistant and will take you about 5 and a bit hours depending upon how you drive and which route you take. Look into hiring a "wicked" camper and that will be cheaper and easier for both accomodation and wheels. You can get them from Syd, Mel or Canberra. Or you can buy my bus off me and drive from Perth

only 56 hrs driving each way

Cheers mate