Wilson Reserve Park is located South of St. Kilda in Brighton. Here you can camp for free in your car. The place is in a residential neighbourhood, but you can park between the park, so you are away of houses and buildings. The spot is close the Brighton Beach, with is famous for it’s colored huts, which you might want to visit anyways.
Be aware that this is a simple street camping spot – no camping facilities are nearby. Check for parking restrictions especially if you want to leave your car here during the day for a longer time.