The building, also known as Centrepoint Tower, offers the external Skywalk experience as part of a viewing attraction at the top of the building. The Skywalk was constructed in 2005 at a cost of almost $4 million.

"Step out into the open air for a breathtaking 45-minute tour around the outside of Sydney Tower with one of our experienced guides," the site reads. "You can enjoy the views from the internal Observation Deck before and after your Skywalk tour."

The 309m tall tower is "ranked as one of the safest buildings in the world", according to the Sydney Tower Eye's website.

"The striking design has made the tower capable of withstanding earthquakes and extreme wind conditions," it reads.

"The breathtaking 360-degree, sky-high views stretch for more than 80km in any direction."

It has been closed until further notice.

Where to get help:

• In an emergency: call 111
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633, or text 234 (available 24/7) or or live chat (between 7pm and 11pm)
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)