Most of you are applying the UK rules (which are an exception) to the rest of the world and are setting yourselves up for one lovely mid-air. The ICAO world wide standard is: "....except as authorized by ATC, aircraft progressing from one region to another shall make the change in the altimeter setting while within the standard pressure region prior to entering, or after leaving, the altimeter setting region." Capt Claret's outline of the rules in Oz mirror the ICAO standard with a TA of 10,000 and TL of FL110. Here in North America, 18,000 has been established as the TA continent wide (the far northern areas excepted) - nothing to do with terrain by the way. With an altimeter setting beloew 29.92", Canadian ATC will adjust the lowest useable flight level and the TL remains at FL180 but in the US the TL varies by 500' above for every .5" of altimeter below 29.92". Thus an altimeter setting of 29.43" in New York produces a TL of FL185. All of the above ensures that all aircraft operating in the same airspace are using the same altimeter setting/flying the same altitudes.
Imagine the scramble if departing practically any airport outside the UK, having been cleared to your high altitude cruise flight level and STD set, when ATC asks for an "IMMEDIATE level-off due traffic" at any altitude within the altimeter setting airspace and also stops the descent of the conflicting traffic 1,000 above. I challenge any of you to get the "rocket" under control without plastering all against the ceiling, look for the traffic and find/reset to the local altimeter setting. The UK rules leave the altitude separation problem to the controller - their job to keep the aluminum apart - and as I spend quite a bit of time in their airspace, I wish them much luck.