If you're in the Sydney area and believe you saw a flash of lightning or heard its accompanying thunderclap in the middle of a snowstorm, you're not alone. There are multiple reports on social media of the same.
For those of you who haven't already Googled it, indeed it's possible and is referred to (per Wikipedia) as thundersnow.
Though considered a rare occurrence, it does happen in Canada, and notably in the Halifax region. Find out more about this weather phenomenon here.