The Prospective Marriage Visa lets the fiancé of an eligible person live in Australia for nine months. The idea behind it is to enable the couple to get married. The eligible person the fiancé is joining must be an Australian citizen, hold a permanent residence visa, or be an eligible New Zealand citizen.
To obtain a Prospective Marriage Visa you must be 18 or older. You also have to demonstrate the engagement and prospective marriage is genuine. This includes showing you know the person you are marrying, will get married to them within the nine month period, and will live with them once married. As long as you marry within the nine month period you can apply for an Australian Partner Visa which will let you live permanently in Australia.