You are right! Queen Victoria did sign the Commonwealth of Australia Constitution Act 1900 (UK) into law on 9 July 1900. However, covering clause 3 of the Australian Constitution says the date when the Act would commence—of when the Commonwealth of Australia would come into being—would be decided by Proclamation by the Queen. This Proclamation stated the Act would come into effect on 1 January 1901.
Sometimes, a law will create a big change, such as the creation of a new country! Preparations need to be made, new institutions set up and people told about the change. To ensure there is time to make all the changes, some laws have written into them a future start date. This date is either specified (the Commencement Date) or the date is decided later. The Governor-General still makes Proclamations about the commencement of some laws after they are given the Royal Assent.