rust like powder are the spores of a fungus. commonly called rust. There are fungicides for treatments. But check with your local garden centre.

Frangipani or Plumeria are either deciduous (rubra) or evergreen (obtusa, pudica and stenophylla). The latter can also lose their leaves if out of direct sunlight or cold. Given that it is summer in Oz, I think we can rule that out.

The rust causes leaf loss and removing all infected leaves and spraying the plant with a suitable fungicide will help as will keeping the leaves dry. Unfortunately the plant may have to live with the rust as it is airborne but keeping it dry and removing infected leaves will help.