Even adults? You're in Australia, I presume. Is that a local, state or nationwide law?Where I live everyone is required by law to wear a helmet.
Yes. Required to wear by anyone riding a bicycle since 1990 in my state and other states followed shortly after.
http://en.wikipedia.org/wiki/Bicycle_helmets_in_Australia
Some exemptions apply and apparently in 2014 in QLD you can have an exemption on religious grounds! WTF that is I do not know.