Visitors got a shock when the glass beneath them on the Sky Tower's famous Observation Deck began to crack.

However, officials say it's superficial, something that's happened after 22 years and 10 million pairs of feet walking over it.

The deck is made up of three panels of glass, two of which are protective and a third which is structural.

A spokesperson says the main glass panel is 1.5 inches thick, and is as strong as the concrete around it.

They are happy the first layer did its job and protected the other layers, and they will be replacing it in due course.