I'd consider selling the investment property but that's just me. With the market as it is you may well pay off the 105% you owe and you will still have your main residence worth a heap. Picking the top is a mug's game but surely Sydney is somewhere near the top now? In fact I honestly don't know how people survive in Sydney. If the median national income is 57 grand and a 1 bedroom flat costs about half a mill it has me buggered how it isn't going to all end in tears.
But hey what do i know i thought that 5 years ago and look where we are now.