To be honest even WITH a referendum the Supreme Court might need to rule on this one, though if there is a referendum to back a change then it is pretty strong ammunition to guide the Supreme Court's recommendations.

Our Constitution doesn't say much about referendums and they aren't included in any of the formula for changing the Constitution, as laid out in the Constitution Act of 1982.  Technically there is no legal reason why a referendum would be required, but as you note without one it is all but guaranteed to go to the Supreme Court who would, in my opinion at least, almost certainly state that a Constitutional Amendment, following the "7-50" formula, is required.  We can't just have the voting method changed through a simple act of Parliament, that would set us up for massive electoral abuse.