The Constitution is the fundamental law of Australia binding everybody including the Commonwealth Parliament and the Parliament of each State. Accordingly, an Act passed by a Parliament is invalid if it is contrary to the Constitution.
The list of powers given to the Commonwealth Parliament does not expressly refer to many subjects including roads but it can, under its external affairs power make laws necessary to give effect to an international agreement.
The sections of the constitution which could be applied to national helmet laws are below.
Under these powers the Commonwealth could legislate to enforce international standards in all states.
There are other sections of the constitution which could be used to argue that the State regulations are invalid.
This is of course open to interpretation and there are several tests which have been applied in high court cases. One test is the test of exclusivity which looks at whether the commonwealth law is intended to be exclusive and looks at the intent of the legislation.
In the case of helmets it is reasonably clear that section 109 would apply and the State Regulations would be found to be invalid to the extent of their inconsistancy with commonwealth law under the test of exclusivity, the reasons are below.
The intent of Commonwealth helmet law was to make use of helmets when riding motorcycles on public roads mandatory. The States agreed and implemented it into State Road Rules leaving the definition of what constitutes an approved helmet to the Commonwealth. The implementation of a mandated standard for the sale of helmets was only done to close a loop hole which allowed people to unknowingly purchase helmets which did not meet the national standard mandated for use.
For over 20 years the States allowed the Commonwealth to set the national mandatory standard for helmet use on public roads. It has only been in the last 6-8 years that the States have moved to write their own standards into their Road Rules.
The evidence which could be presented in any high court case is substantial.
In effect this section is saying if a helmet is legal in one state then you should not be fined for wearing it in another state.
This section reinforces section 117, it ensures court decisions and precedents set in one State apply in the other states.
The constitution can be read or downloaded from aph.gov.au and makes interesting reading.