For a 3 months visa there is no need for the clearing officer at the Australian Embassy to ask for a medical to be done, however, if you apply for a longer Australian tourist visa including the 6 months and 12 month visas, then your Thai partner will be asked to go and have a Medical done in Thailand as standard practice. After the application has been submitted, your Thai partner will be asked by an Australian Embassy Officer to  go for the medical and given a unique medical number. This is also used to book the appointment for your Thai partner. Use the service of Key Visa Thailand and save yourself the trouble.