"It keeps the hailstone in the cloud for a bit longer, and then the water that’s surrounding the hailstone starts to freeze on the stone," he said.

"As the water starts to freeze on it, it starts to form icicles, so you start getting what appears to be clear ice around it, and then as [the hailstones] start to freeze on each other, you start getting some rather interesting shapes."

Mr Ryan said in a "normal" hailstone the hail does not stay in the cloud for very long, so it is not able to grow very large.

"It tends to just fall quite rapidly to the ground [and] we end up seeing those smaller, one-centimetre sized hailstones," he said.

"With these stones, because they’re able to stay up in the cloud system for much longer, these things are able to freeze on the stone itself, which gives it these unique shapes."

While Mr Ryan said most hail would fall at a similar rate, the larger ones "will obviously have a far more significant impact".

The hail had such an impact the Insurance Council of Australia has declared the event a "catastrophe".

The ICA said that by 6.30pm on Thursday, 15,000 claims worth about $18 million had been lodged, and more were expected on Friday.

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Mr Ryan said the largest hailstones recorded by the bureau were seven to eight centimetres in diameter.

A number of hailstorms rolled through Sydney on Thursday, he said. Some were quite fast-moving and some systems produced quite large hail.

"Then with some other storms that were behind it we saw hail that was one, two centimetres in diameter and there were another couple that were five to six centimetres, so it really does vary from system to system."