The law generally states (for all states as far as I am aware) - no more than 6 forward-facing white lights (that aren't riding lights) and they must be no higher than 1400mm off the ground.
It means that technically speaking you can't have ANY light up on the roof. Technically.
It would have to be a copper that had just had the bank foreclose on his mortgage, his wife hasn't put out for a year, his girlfriend gave him the clap, he was busted by the superintendent wanking off in the empty cells to the sports page and he'd tried going through a McDonalds drive thru to get a free cheeseburger and copped a pair of wasps in the open window while he was rattling off his badge number and the reason why the bimbo should give him a freebie. That kinda guy would see the lights and go "I'm going to make someone's day as bad as my own!"
Seriously, you'd have to be doing something REALLY idiotic for a copper to go you for them. Insurance would have to prove that the existence or use of the lights were the cause of an accident.
So technically they're illegal, but if you're being sensible then you're unlikely to suffer any grief over them.