Why are you going for the OA visa you have to have health insurance and a police check if done in home country better getting the O visa no health insurance no police check.
If your worried about getting sick get travel insurance for up to 364 days and renew when back in Oz next time it's about a ¼ of the price of health insurance. And your best off to do the 800k in Thai bank monthly payment can take up to 3 years to get dates right