Looks like a Mimosa. They can be trees or shrubs. They can also be invasive if you live in a warm climate. When I lived in NY we had one called Golden Rain Tree. It grew as a tree but was not a large tree. Very pretty in August when most trees don't flower.

Living in Florida, we have various types. They get pretty flowers but large seed pods so pop up all over the place.

None are large trees.

Jane

It could be Albizia julibrissin, sometimes called Mimisa, but Australia has many trees in the Fabaceae family which would look similar. Did it just pop up in your garden? If so can you find the parent? Mimosa is a common name applied to many different species, including Acacia dealbata. Golden Rain Tree usually means Koelreuteria paniculata. Your seedling us definitely not that.

It could be Jacaranda.

If in Australia, it could just as easily be Acacia as it could Albizia. There are over 1000 species of acacia native to Australia and most will have an appearance that looks like that.

Would need to see any flowers to confirm a genus.