Ooh - I have first-hand experience with this! I had green, but needed to stop because a pedestrian was crossing the intersection illegally somewhere in Hawthorn. I was over the line, but not 100% over the line. When the pedestrian cleared the way, I continued and the camera went off. Your whole car needs to be over the line when the light turns red. The flash goes off the same millisecond, there is zero grace on that.

I challenged, and had to go to court. Explained what happened and the magistrate and he agreed that I was not breaking the law. I can’t find the outcome term online now - I’m at work and don’t have time to research it. I can explain what happened in layman’s terms though:
When you challenge the offense, you are saying the police made a mistake. That you shouldn’t have been issued an infringement. The police will fight you in court over that.
In my case, I was given a verdict that basically meant “person is guilty but there are extenuating circumstances”, and I didn’t need to pay the fine etc. It did cost me a day off work, though

If you are saying your front tyres were over the line, so you’re not guilty, I don’t think that’ll fly. Your whole car has to be over the line when it turns red. In my case, they could see I was doing like 5-10km/h when I went over the line, I obviously wasn’t zooming through. So while the photo didn’t show the pedestrian, they believed me.