It doesn't. Manufacturers and importers are a benevolent bunch and love spending $30-50K on retesting helmets to get a nice shiny sticker from the Australian Standards nazis.

However, in the event that my head was being run over by every wheel of a road train you can be sure that I'd only trust an Australian Standards certified helmet

[EDIT]However there is a claim on the Bicycle Helmet Safety Institute website that the Oz Standards testing is superior to US testing in a number of areas, but it does not elaborate

[ANOTHER EDIT]Deep in the bowels I found this unvalidated comment relating to the recent recall of Trek helmets: "Opening up huge vents automatically means reducing the impact foam, and that in turn requires making the foam that does remain very dense and hard. It becomes brittle, and must have internal or external reinforcements to hold together. The hard foam spots can put more localized load on parts of the skull as well. The Australian standard tests for that, but we do not in this country (ie. USA)."

[FROM WIKI] "The CPSC and EN1078 standards are lower than the Snell B95 (and B90) standard; Snell helmet standards are externally verified, with each helmet traceable by unique serial number. EN 1078 is also externally validated, but lacks Snell's traceability. The most common standard in the US, CPSC, is self-certified by the manufacturers. It is generally true to say that Snell standards are more exacting than other standards, and most helmets on sale these days will not meet them (no current Bell brand helmet is Snell certified, some Specialized ones are - the Snell Memorial Foundation website includes a list of certified helmets)."

http://www.smf.org/certlist/std_B-90...90C_B-95C.html