You get 7 days stay on a General Declaration. You can then apply for an extension up to 30 days, but beyond that you are expected to pay import VAT at 14% on the value of the aeroplane. Its a nightmare.
You may have noticed that a few years ago airports like Lanseria had loads of foreign registered aeroplanes brought in for maintenance but nowdays there are very few? The change is all thanks to the work of Customs and their combined IQ of 36.