The Prospective Marriage Visa is for people who are engaged to marry an Australian and wish to enter Australia to marry the fiance(e). This visa can only be applied for from outside Australia. They do not have to be married or be a de facto couple to be eligible. However, the couple must have met in person at least once and intend to marry in Australia within 9 months of visa grant. After the wedding, the next step is to apply for the Partner Visa while in Australia.
On the other hand, the Partner Visa is for people who are already married, in a de facto relationship or in a registered / civil relationship.