Regardless of whether you have a Thai tax file number or not, under the Australia/Thai DTA Oz CANNOT legally tax you, even at non resident rates as long as you are a resident for Thai taxation purposes (viz 180 days or more per calendar year). The DTA stipulates that ONLY the country where a person is a resident for tax purposes can tax that person, viz. Thailand. All you need to be able to do is prove to the ATO, preferably using a tax accountant who has full knowledge of the DTA, that you have been a Thai resident for tax purposes for those 4 or 5 years and submit a claim for a FULL refund of all taxes paid to the ATO during that period. Earlier this year I telephoned a tax accountant in Melbourne who specialises in DTA's, with regards to my Oz Age pension to be told that even though it was tax free in Oz it was assessable income here in Thailand, and during that conversation he told me about one of his clients who had been living fulltime in Thailand for the previous 15 years and being taxed by the ATO at the Oz non-resident tax rate of 32,5% on his age pension. The accountant lodged a claim with the ATO making them aware that under the relevant section of the DTA that the ATO could not legally tax their client on any income as he was aThai resident for taxation purposes. The client received a FULL refund of the tax paid over those 15 years.
The thing to keep in mind is that as per several posts by Mike Lister, there is a full range of deductions that we can claim to reduce the actual assessable amount thus reducing the amount of tax payable if any. In my case those deductions will reduce the annual amount of my assessable income (age pension only) to below the 120,000 baht level thus resulting in my still having to lodge a tax return BUT not having to pay any tax whatsoever. The other step yhou should take, via a tax accountant, is to have the ATO notified that your last tax return was your final one as you would not be returning to Australia.