"The green colour is coming from the gas that is coming off the comet," he said.

"There is a bunch of ice on it and methane - it's essentially like a dirty snowball and so when it goes around the sun it melts ... and is a steamy, stinky green glow."

Hours later shooting stars are set to flash across the sky as Earth passes through the tail of the 3200 Phaethon asteroid.

They'll look impressive from the ground but the falling stars are actually just small rocks that have broken off from the asteroid before burning up in the Earth's atmosphere.

"They're about the size of a grain of sand, or even a small pebble and they're travelling tens of thousands of kilometres an hour," Dr Tucker said.

While observers will need a pair of binoculars or a telescope to catch the Christmas Comet, the meteor shower will be visible from anywhere in Australia, even major cities, so long as it's a clear night.

"It's very accessible, you don't need anything special, you just need the night sky," Dr Tucker said.

Friday's forecast:

Sydney: Mostly cloudy conditions and low mist, possible breaks.

Melbourne: A cloudy night with scattered thunderstorms.

Brisbane: Mostly cloudy.  High chance of showers developing.

Adelaide: Cloudy but possible breaks.

Perth: Mostly clear evening.

Hobart: Mostly cloudy conditions.

Canberra: Possible breaks but mostly cloudy.

Darwin: Partly cloudy.

AAP