I don’t know about Australia (land of sunshine and sharks :D), but here in the good ole U.S. of A., we have several ways of dealing with such issues. Oftentimes the street number roughly corresponds to how far down the block the house is located, i.e. my house is numbered 521, so it’s down from the end of the block about 20% the length of the block. As more houses are added, they get street numbers roughly corresponding to where they are down the block. Another way this is dealt with is by leaving room between numbers, i.e. instead of going sequentially 1, 2, 3, the houses are numbered 1, 5, 9, etc, which leaves room for additional numbers. A third way of dealing with street numbers when things get crowded is to add letters to the end, i.e. 521A and 521B. Another way I’ve seen (mostly in the eastern US) is to add 1/2, i.e. 521 1/2. And finally, there is the option as you mention of changing the number, although I’ve personally only seen this in areas where a small rural area starts turning into suburbia.