Vietnam is one of the interesting destinations because of many beautiful landscapes, diverse cultures and special foods. Therefore, this country has been attracting many visitors to stay here for a long time to explore as much as they can as a temporary residence. 

For your question, it is possible to stay in Vietnam for more than 6 months. However, 6-month Vietnam visa is not applicable for Australian citizens as the following table:

Therefore, Australian citizens can just apply for Visa On Arrival with single or multiple entry to stay in Vietnam for not more than 3 months. Then, it is still possible to extend your visa. Although 6-month or 12-month visa is not applicable, but you can choose this way to stay longer in Vietnam.