Green lightning?

Last night there was a huge storm in Sydney. Given how hot it was all weekend, that's not terribly surprising. Was a pretty cool storm, going from about 23:00 until well after midnight, as it kept swirling around. We'd be just dropping off to sleep when another crack of thunder would shake the house.

Anyway, we're sitting out the front watching the cool lightning displahy when both Holly and I spotted a very bright, green glow to the North-West. It was green lightning, but perhaps behind clouds.

Searching online, I've found other references to green lightning, but nothing explaining what causes it. Then again, it seems the causes of ordinary lightning are, amazingly, still open for debate, let along exotic types of lightning.

So anyone know anything about green lightning?