Western Sydney University is a universitywide partner in Sydney, Australia. You can study at Western Sydney University through the AEN program. There is no direct (bilateral) exchange agreement with Western Sydney University, so you will have to apply for AEN if you want to study there.
The AEN program (Australian European Network) consists of a group of 5 Australian universities. Utrecht doesn’t exchange with these universities directly, but via AEN. You will have to choose AEN as the option in Osiris. If you are applying for the AEN program, you need to mention the top three AEN universities of your choice in your motivation letter, which needs to be uploaded in Osiris. This means there is a chance you will not be placed at your preferred university, but at your second or third choice university instead.