Greetings, welcome to the Forums.
This is the second Forums thread about Elm (Ulmus spp.) damage from Australia in the past couple weeks. I understand Australia is experiencing an epidemic of Elm Leaf Beetles (Xanthogaleruca luteola, Chrysomelidae). They were first reported in Australia in 1989.
There are treatment options that involve spraying (or injecting) pesticides on the Elms, but I don't recommend chemical warfare over an ornamental plant. Sometimes an introduced predator can control these outbreaks, but I doubt efforts would be made to introduce an predator to protect a non-native tree, unless it was considered economically vital.
Judge how severe and unsightly the damage is, and decide if you can live with it or want another tree. Perhaps plant a native species, They will come with its own insect fauna, but it will include predators as well as herbivores.