you are not working inside Thailand. You are working remotely, and Thailand trusts you to file the appropriate tax in your home country, or the country you remotely work in, on the money you bring into Thailand. You are trying to overthink this.
This will better let you understand your tax liability. In order to file taxes in Thailand you must have a TIN, taxpayer identification number. Go to your local revenue office with your DTV, and no work permit, and try to get a TIN. That will answer your question about paying income tax in Thailand. It is that simple.
they can also call you in for an interview in some cases, and if you are out of the country that can be a problem. At some point when they review your application they also check with immigration to see if you are in fact in the country.
Okay, if you are serious about this permanent move to Thailand, take it in steps, and this is a little more detailed to what everyone has said so far. First, set up separate bank accounts in Australia for both you and your wife in each of your names, with at least the equivalent of bt800,000+ each, not a joint account, this makes some future details much easier. For your next trip here go onto this website, and apply for the type O visa based on retirement, and each of you do this so you each have separate visas, and why you need two bank accounts. This gives you each 90 days in Thailand when you enter. You've got 90 days to use it once it is issued, and is bt2000. That this gives both of you is the needed expat visas to open your Thai bank accounts without having to use an expensive agent to do it. Sure you could come in on a visa exempt 60 day free entry, but you can not get a Thai bank account on any tourist entry anymore by yourself, and in the future you will both need your own type O visa extension (one year at a time), so you both need your own bank accounts to do that, as not every immigration office allows joint accounts for both your retirement visa extensions. Also, it gives you the ability to use electronic payments here with no problem on your first vacation. When you decide where you actually want to live, and long term, your Thai bank accounts will have been taken care of, as they do not disappear, and you can get another type O visa for the initial 90 days living here, and the time to get your money transferred into Thailand for your long term visa extensions. This is the easiest long term visa extension to get. As soon as your bt800,000 baht has been in your accounts for at least two months, you can apply for your one year extensions, at bt1900 each.
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Foreigners can not own land. You can only lease it, for upto 30 years. You can own condos though. Bangkok is very good to base out of. Best medical is in Bangkok. Best food selection is in Bangkok. And, you can go most anywhere in South East Asia and all of Thailand, in just a few hours. It is very nice living here. Also, make sure you have your health insurance worked out here, as the older you get the less choices you have. Private hospitals are the most expensive, as they are for profit. The government, and university hospitals are top quality, and about
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as expensive. Don't be fooled by promotions by some of the private hospitals, all doctors speak English.
Anonymous participant you seem to have money so just buy the Privilege Visa, that used to be called the Elite Visa, from bt600,000 for five years, to bt3million for 20 years, or you could go the actual Investment Visa route through the BOI, still with a minimum bt10million in approved investments, and now does allow approved condo purchases, as part, or all, of the minimum financial requirement.
As long as you follow a pattern of an actual tourist, and not someone trying to live here long term as a tourist, you'll be fine. That involves staying at least 5-7 days or longer in other countries surrounding Thailand, not the same place everytime either. There are lots of long term tourists that base out of Bangkok, traveling all over South East Asia, for years. As soon as immigration decides you are trying to stay here long term as a tourist they will refuse entry.