Not just that, but the Dutch Olympic cycling organization mandates wearing a helmet.The NL information is buried somewhere in here:
The Netherlands had an average 2,150 cyclist hospital admissions for head/brain injuries per year from 2003-2007. Per-capita travel by bicycle was 2.5km/day.
Australia had about 1,122 serious head injuries in Australia in 2005/06 for cyclists. Per-capita travel by bicycle was 0.1km/day in that time period.
Australia has a 25% higher population, but even still it's quite clear Australia has far higher number of hospital admissions per km cycled than Netherlands despite most of Australia having mandatory helmet laws at that time. These injury rates were down significantly from the '80's before helmet laws were in place.
I don't have Canadian (or Ontario) numbers but I expect they're much closer to Australia's than NLs.