Last time I looked cost sharing in Australia was perfectly legal for private flights. So are free flights if the pilot so wishes and always have been.
All you need do is make sure the passenger/s is aware it is a private flight and also should advise them of the insurance cover. It is then up to them if they choose to proceed or not. No different to jumping on the back of a motorbike,speed boat,or bungy jump for that matter.
It does not need to get any more complicated than that unless it is a commercial operation for hire and reward.