You will not pay tax if you are in Thailand for less than 180 days per tax/calendar year (unsure of which applies).
Savings, if that's what you will be living on is not income, a pension or rent from a property however is an income. If you have an income it is subject to income tax usually within the country in which it is sourced, in your case Australia.
Thailand has "double tax treaties" with 60 countries which includes Australia, therefore if tax has been paid in Australia you won't be taxed again on the same income.
To pay tax in Thailand an individual would need to be registered as a tax resident, how that's accomplished I'm unsure but a degree of permancy would be required for this to be progressed.
There are a couple of interesting articles which provide an overview of this matter and I'll include them below. I hope they help answer or assist you in some way.
there's nothing better than seeing someone asking for help and advice only to be berated by a complete stranger who offers no help or advice. It gets better when they later reply with an abusive and insulting comment trying to aggregate and agitate the OP. So clever and well thought out. Now please go back to your bongo drums and leave the grown ups alone or better still leave this group as your comments are not welcome.
The cost is likely to be associated with your class of ticket not so much the agent. When booking you're normally presented with a range of prices, each have their own conditions such as luggage allowance, air miles etc and one of those conditions is the cost of changing flights.
Yes it works with a Thai SIM. I deactivate my UK which is my primary number and the one I had when setting up wise. Whilst I'm Thailand I use wise to transfer money from the UK to my Thai account with only my Thai number active.