Not sure what kind of come-a-longs they have down under but they can have very long lengths here in the USA. I happen to have a chain come-along that has a 30 feet of chain. Granted that I modified it by removing the original chain and putting in a longer one, but I could theoretically put as much chain in it as I wanted. It came with a 20 foot chain when I bought it and can be bought with anything from 5 to 20 feet of chain.Nahh.....a come-along doesn't have enough length to go up a 20 foot stair length....it'is a short lift device, either a chain or wire rope type.
https://www.cmworks.com/Hoists/Lever...chetLeverHoist
https://www.mfrexpress.com/chain-com...gaAuAQEALw_wcB
I also have a cable come-a-long that came with 20 feet of cable. 20 feet of pull at 1 ton, or 10 feet of pull when double line.
Residential stairs are only about 16-18' long unless you have 12 foot ceilings