The wearing of UN ECE 22.05 helmets has been legal in NSW since just before Christmas 2015. For more information see http://roadsafety.transport.nsw.gov.au/stayingsafe/motorcyclists/motorcyclehelmets.html.
For maximum protection in the event of a crash, it’s important that the helmet stays in place. The above link has advice on choosing a helmet that is a good fit, particularly the roll-off test. Our advice is not to purchase a helmet you haven’t actually tried on for fit and comfort.