The Australian Fiancée Visa for Thai girlfriend is explained here. Note that there is an Australian spouse visa or partner visa and there is an Australian fiancée visa for Thai. The spouse visa is called the Partner (provisional) – Subclass 309 visa while the fiancée visa for Australia is called the Prospective Marriage (temporary) – Subclass 300 visa. See the article on the marriage visa for Australia if you are already married to a Thai national. This temporary visa for your Thai fiancée is explained below. The visa for a Thai to Australia is explained below.
Australian Fiancée Visa for Thai
Much like a US Fiancée visa the Australian fiancée visa also has a time limit. Once the visa is granted and your Thai fiancée enters Australia you have 9 months to get married. The visa application process can take up to 12 months to complete and you Thai fiancée with have to have a police clearance certificate as well as a health certificate to complete the process.
Most delays tend to be where documents have not been submitted or are incorrect. If there is no other reason for a fiancée visa then you can get married in Thailand and if you have lived together already as husband and wife in Thailand then you can already register your marriage in Thailand for a spouse visa. Most expats in Thailand returning home tend to have lived together with their Thai fiancée for years while on a Thai work permit. Take proper legal advice on your options when it comes to immigration.
Fiancée Visa for Thai
Note that she must have entered Australia at least once on the fiancée visa for Australia and once married she will have to apply to 
have her visa status changed to a Partner (provisional) visa. As stated above that if you have lived together for longer than 3 years in Thailand or at least 2 and have a child then the partner visa application would make more sense.
If you get married while the fiancée visa is being processed then again the Embassy will need to be notified as the visa application needs to be changed. The same health certificate which was used for the original application can then be reused again. The Embassy will have the final say on this issue.
Note that if you are taking depended with you to Australia on your fiancée visa then there are limitations on them entering Australia.
These two limitations are:
- 1. They cannot enter Australia before you;
- 2. They cannot get married before they enter Australia.
See also the page on this website about the documents needed for the application process while in Thailand. Always take legal advice before you start the process. This as the marriage laws pertaining to property are not the same in the two countries. Thai property laws in marriage are not the same as Australian property laws and you might want to consider a prenuptial agreement drafted for Australia while still applying for the fiancée visa.
Children are a complex issue when travelling across international borders. If your Thai fiancée has dependents. Then they will also have to obtain a proper visa for their move to Australia. If the children are not yours and your Thai fiancée is divorced. Then you will need copies of the divorce decree to see who the courts gave parental rights too. Take legal advice before you start the Australian fiancée visa for Thai process.