Hi. I think in terms of ranking they are both top world with National University of Singapore is rankied 11 in topuniversities and University of New South Wales ranked 45.
The big question is where do you want to study? in which country? I think Singapore and Australia are totally different.
Also do you intend to stay in the same country of your PhD after finishing? How is the job market in your prospective field? How easy to get a work permit in each one (assuming you are not a citizen of the two contries). In my opinion, research wise, both are sufficiently top. The country is the real choice.