Should I be concerned about new helmet laws?
Summary: a helmet that meets the new standard is going to protect your head better than one that just barely meets the old standard.


Old Law: AS/NZS 2063:1996 (PDF); unfortunately just the start, the actual full text of the law is behind paywalls.
New Law: AZ/NZS 2063:2008 (PDF); also need to pay to get full text
some stuff about US rules for comparison: CPSC bicycle helmet rules (PDF)


Summary of differences from the preface of the new regulations:


Something about "projection requirements" from the horse-riding helmet regulations added in. This is probably about stuff sticking off the outside of the helmet (such as a visor/peak).
Update to what headform is used for testing to match an international standard
Reduction of maximum allowed force to headform in test from 300g to 250g.  For comparison, 300g is the US requirement for adults, but 250g is the requirement in the US for children. I think it's a similar impact test.
Update to test of retention system (straps)
A new test for "peak deflection" added in, which I believe is another test of the retention system (straps), but measured differently.


Also, from the stuff I saw, a helmet meeting the old standard will still be perfectly legal to own or wear, will still be allowed in organized races for a while, etc.

I think the 300g to 250g change is the big one.  The g forces imparted to your head in a collision directly translate to how much damage your brain is going to take.  They're taking a weight shaped like a human head, strapping it into the helmet and dropping it onto various "anvils" and measuring the impact. This is a laboratory test of what would happen to your head during a collision wearing that helmet.

Note that the tests are simply pass or fail.  It's entirely possible that there's helmets certified to the old standard that would pass the new standard (or parts of the new standard) but those models weren't tested to it.

Personally, I'd be likely to pick up one of the old standard clearance helmets and not worry about it. Maybe get something a little nicer than I'd normally choose to afford. If I was planning some riskier riding I'd probably choose something certified to the new standard. However, I'm opposed to adult mandatory helmet laws, so my personal opinion should be taken with a grain of salt.  My personal opinion is that it's 90% silly bureaucracy and 10% actually making your head safer.