Graduate studies (phD & Masters) after a double degree I am planning on doing Bachelor of Computer Science - Bachelor of Mathematics at University of Wollongong, Australia. Do you think I will be able to do a phD in Mathematics by studying Mathematics as a double degree?
TLDR -- Yes.

Long answer -- A Ph.D. in Math is possible for a student who has double majored in Math and CS. A double major (at least in North America) consists of almost all the courses for both degrees. Taking some CS courses in undergrad is a good idea anyways for most math career paths.

With that said, Alexandros is right to point out that this is a bit premature. High school (and early undergraduate) math bears little if any resemblance to the math that a Ph.D. requires.