The passing of the law led to a very large step change in helmet wearing, from less than a third to nearly 100%. The improvement in cycling casualties figures should therefore be very easily detected.
You've quoted the "nearly 100%" figure before, please show me the data because I can't see it. Sample states below:
Victoria Police data, "Of cyclist injuries recorded between 2010 and 2013: 78 per cent were recorded as wearing a helmet; 5 per cent not wearing a helmet and 17 per cent were unknown." https://www.tac.vic.gov.au/about-the-tac/media-room/news-and-events/2015-media-releases/25-years-of-mandatory-bike-helmets-in-victoria
New South Wales, "Adult helmet use rose from 28% to 77% at the time that the adult law came into force, and then to 85% upon enactment of the child law. Child helmet use remained at 31% until enactment of the child law when it increased to 77%."
"According to a statement made in the New South Wales Parliament in 2005, helmet use was then much lower than when the helmet law was enacted (Hansard NSW, 2005). The Roads and Traffic Authority had continued to monitor helmet use but the data had not been released because compliance had fallen." https://www.cyclehelmets.org/1109.html
Queensland, "For the first 18 months, the helmet law was not enforced. Helmet wearing increased initially, but 17 months later was only a little higher than pre-law (King and Fraine, 1993):
Cyclists wearing helmets:
Primary schoolchildren 72%
Secondary schoolchildren 21%
Commuter cyclists 37%
Recreational cyclists 22%"
https://www.cyclehelmets.org/1104.html#154
Regards
tim-b