The wearing rate went up to close to 100% from about a third...snip
"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 (Victoria Police data)." 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
That isn't overall helmet wearing rates, clearly, but 23 years on from the introduction of the mandatory helmet law and and there isn't universal use of helmets in the State of Victoria. Neither the fines levied nor fine levels translate to compliance, as demonstrated with mobile phones here
Regards
tim-b