He answered someone who asked, he wants a visa to move from Australia to Thailand for him. So I'm assuming he wants the bank account to support that, which he will need.
If the 800k is brought into country in the tax year before you become tax resident, (first year june onward), it would be totally tax free, as first tax return is not due for upto 18 months). If it is brought in the same year as becoming tax resident, then it is subject to tax. But depending on dta with your country, and when the money was initially earnt, it can usually be credited out or exempt (pre
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/23) and no or little tax owed once other allowances are considered. But you only pay tax after filing a return. Paying tax and owing tax are two different things.
A joint account will not help with your future visa application. It will mean that you would have to show twice the required amount as it is understood that only 50% of the monies contained are yours and the other 50% belong to your partner, regardless of who banked the money when proof of funds are required at visa time.