I googled for the cycling laws in Victoria, and found
this site.
- Police can stop bike riders and issue a fine or a warning for not wearing a helmet.
That's all it said about fines. There is a pdf brochure attached, that says
There is a fine for not wearing a securely fitted helmet. For more information visit
vicroads.vic.gov.au/bicyclerules
That link, of course, doesn't work but if I navigate to the relevant page on the site, it's the same one as above that doesn't mention fastening.
There is nothing in Australia like the Highway Code, so after finding no definition of securely fastened I found
the legislation - don't bother downloading it, it's huge. Guess what? There is no definition there either, just stuff like this (and similar for motorcycle helmets)
A passenger on a bicycle that is moving, or is stationary but not parked, must wear an approved bicycle helmet securely fitted and fastened on the passenger's head
So how can the police decide what is securely fastened or not? And how difficult should it be for someone to find this out?
Note, it used to be common to see helmets not fastened at all in Australia, which I think was legal at the time.