I understand. The Australian Government is very sensitive to people immigrating to Australia through marriage, especially from countries such as Thailand. Mainly because this process has been abused in the past. The Australian Government needs to be satifisfied beyond doubt that the relationship is legitimate. Getting married online would create doubt in Australia as to how legitimate the marriage is. Not to mention, it's currently not an approved way for Australians to marry.
are you Australian? They will be interviewed as part of their application in Australia. The Government representative will ask you about your marriage and relationship to determine if the marriage is legitimate. They will find out the marriage occurred online in a third country. You do not want to give the government any reason to decline your application.
like I said, I'd get legal advice. Should they seek residency in Australia for his wife, the application will show that they were never in the US. This would then beg the question as to how they have a US marriage certificate if they've never been to the US. Then if it's unlawful to be married online in Australia (which it is), it could invalidate their marriage in Australia and undermine their residency application. Having worked for the Australian Government and having also been through this process, I know they go through applications with a fine tooth comb, especially considering Thailand is a high risk country for Australian immigration. Not sure how Thai authorities would view this though.
not necessarily. Read the second bullet point. As you cannot get married online in Australia, it is likely an online marriage in another country would not be recognised in Australia. I'd be seeking legal advice.