If you're thinking of a picturesque island a bit less developed than Phuket, why not check out Koh Tao over in the Gulf of Thailand. You can get from Phuket or BKK to Koh Tao in less than one day and it's well worth the effort.
With a 200AUD per night budget, check out the quality Sairee Beach accommodation at Koh Tao Simple Life Resort.
Or if you really want to go rural, check out Koh Lipe or Koh Payam off Thailand's west coast - these are the new upcoming islands that are still off the beaten track which needless to say, also means trickier to get to.