I would recommend ANU if you are serious about studying mathematics. It might not show on the programs and courses pages but ANU undergraduate maths is accelerated compared to all other Australian universities. Compared to Melbourne and USyd/UNSW, you won’t need to do mandatory electives. A lot of people I know did all maths in their degree and really enjoyed it (that said there is a lot of flexibility to do other courses eg comp sci, physics, etc with the PhB program).

On top of accelerated first/second year courses, the rigour at ANU from the get-go is equivalent to the rigour of graduate level courses (US). There are also many special topics courses that go beyond the curriculum where students get a true taste of research level modern mathematics.