sounds like you may have a ground problem somewhere, this happened to my sisters mate she took it to her auto elect and got it sorted.
firstly check that all your fuses are ok, secondly try taking the globes out and putting them back, again i know but thats the process or elimination
have you had anything installed recently that may of caused this to malfunction (upgraded alarm systems, air horns, aftermarket electrical components)
if not then its probably your Brake switch, the brake switch is located at the top of your brake pedal, go to holden get your $13 switch (cheapest available) and then fit it or take it to an auto elect to have it fitted