Water gardens need to be maintained as some plants will thrive all year round and some may be seasonal. When plants get too big their root system could take over and become invasive. It is best to pull out plants before they get too big and split the root ball, then replant half in the pond and give half to a friend. This should be done once a year to once every 2 years. There are many water plants to choose from and can be sourced locally through Suncoast water gardens (www.suncoastaustralia.com).
This is a trade nursery so is not open to the public but they will send plants via online orders (these will be bare rooted for delivery) or they can refer you to a local retail outlet. Check out their plant list and pictures of all their water plants on their website.