If the first two colonies to build railways both standardised on 5'3'', then maybe the railways linking all the state and territory capitals would be Irish rather than English gauge. Australian rail vehicle manufacturers would be tooled for that gauge, and not English or Cape gauge. That New South Wales changed plans regarding track gauge after my home state started construction must have been an unpleasant surprise to people of the time.
Many decades later, a genial engineer called John Joe Crew Bradfield designed an electrified suburban railway network for the Sydney metropolitan area. Like all other Australian cities now with a population or over a million, Sydney has surface rail serving its C.B.D and has had it since the railways came to New South Wales. He saw that that the existing railway network could be electrified within the metropolitan area and extended underground in the most heavily developed areas, with the same railway network serving both the suburbs (except the east and inner west), mostly in cutting and on embankments, and in tunnels in the inner west, C.B.D, and eastern suburbs. This would give a user experience far better then had been seen in any of the elder global cities which were heavily developed before the railways came, these ending up with metro style rail separate from regional rail.
Apparently, he did fight for a very large loading gauge, but not a wider track gauge to match. Is it true he saw no reason to use English gauge other than interoperability with existing English gauge lines?