Some countries have very strict requirements for granting visas, and some do not allow HIV positive people to enter their country at all. Other countries may allow you to enter for short stays, such as on a tourist visa, but will have different requirements for longer or permanent visas.
If a visa application form asks you about your HIV status, then you are legally required to disclose it. If you don’t disclose, and the authorities in your destination country find out, then you can be refused entry, or deported if you have already entered the country. You may also be fined or possibly even imprisoned for making a false declaration.
For the most accurate information on visa requirements for a particular country, you should contact their diplomatic mission in Australia.
Source: HIV/AIDS Legal Centre, Sydney http://halc.org.au/publications/guides-to-hiv-and-the-law/