What about, for the sake of argument, we flip everything upside down, and see how the state government would go defending an 'imaginary' charge of 'Deprivation of Civil Liberties' as a result of excessive helmet promotion followed by mandatory helmet laws - with a defence of necessity!
As we all know, for the defence of necessity to be available, the circumstances must create a situation of necessity whereby a person commits a crime in order to avoid serious consequences that could arise from the situation. An accused must honestly believe that serious consequences would ensue if they did not commit the crime, and they must hold this belief on reasonable grounds. In addition, the conduct of an accused must be proportional to the consequences sought to be avoided.
1. Necessary belief - so would the government have it?
Could the government respectfully submit that the evidence collected by the Royal Australasian College of Surgeons during the 1980s in a bid to bolster helemt promotion would be important in assisting the court to assess their evidence as a ‘genuine belief’ and not a misconceived notion?
To establish that government belief is based on expert opinion and not a misconceived notion, the government would have to demonstrate that the Royal Australasian College of Surgeons are experts.
Questions to ask would be were the Royal Australasian College of Surgeons experts under section 79(1) of the Evidence Act? Did the Royal Australasian College of Surgeons have specialised knowledge based on their training, study and experience with regards to the mechanics of brain injury and design of devices for anatomical modification ? Did the Royal Australaian College of Surgeons' evidence contain the required elements of knowledge in this area as distinct from belief, and was their knowledge specialised in this area rather than generally held in the community?
Upon the facts it would appear that whilst the Royal Australasian College of Surgeons are experts in the field of surgery, it may be questionable as to whether they are experts in the field of the mechanics of brain injury, design of anatomical devices and traffic engineering.
2. Proportionate response - has the harm inflicted been less that the harm sought to be avoided?
Questions to ask would be whether the conduct of enacting the restrictive Road Rules 2008 Regulation 256 has been proportional to the consequences that government sought to avoid?
It would appear that govenment rationale for the enactment of the Road Rules Regulation 256 was that it was necessary to protect the community from the risk of serious danger to their lives from cycling accidents.
But in the circumstances was it out of proportion to the danger to be averted ('supposed' severe brain injury or death)?
No randomized controlled trials have been done on the subject of helmet safety. Current data comes from two main types of observational study; "time trend analyses" and "case control studies". Most of the literature that mentions helmets and helmet promotion refers back to a small number of these studies, rather than actually providing primary evidence. Notions of common sense, anecdotal evidence and medical supposition would not constitute a proportionate response, and any imagined community benefits from helmet promotion and mandatory helmet laws would appear to have been outweighed by the distinct and damaging disadvantages in the decreased numbers in overall cycling.
3. Imminent peril - the situation must be so urgent and the danger so pressing that normal human instincts cry out for action and make a counsel of patience unreasonable (Perka v The Queen  2 SCR 232)
The legal safeguards established by the Magna Carta and the Bill of Rights (1688), which are received law in Australia, appear to have been suspended. There was no documented imminent risk to the community and there was no emergency - merely supposition. In fact we know that general road safety in the 1980s had been improving as a result of many factors including improved road conditions.
There were reasonable alternatives to the 'Deprivation of Civil Liberties'. Notwithstanding the recommendation by the Royal Australasian College of Surgeons that helmets offered significant benefits in terms of the community and cycling, a committment by government to invest in cycling infrastructure and an intensive education programme for motorists would have ensured that 'Deprivation of Civil Liberties was not necessary - the imminent peril requiring the enactment of mandatory helmet laws was not evident.
Therefore in view of all the facts & circumstances and upon my understanding , I believe a defence of necessity would not be able to be found in an imaginary charge of 'Deprivation of Civil Liberties'.