The easiest way to explain it is that the HOTTAP runs on the same gas that your home BBQ does.
We use (most commonly) 9kg LPG cylinders.
The cylinders used are nearly identical but they're labelled as 'UPLG' instead. The 'U' stands for 'Universal and it's because in New Zealand (and many other countries) the mix proportions of Butane and Propane in the cylinder can vary quite a lot, whereas in Australia it's fairly consistent. It's not a concern and HOTTAP has been tested and approved with a much wider range of propane-butane mixes than would be found.
In the United States
This is called a Propane Tank and the standard size is 20lbs but they can also come smaller.
In the UK
We recommend the use of Standard Propane over Butane gas. Read more on UK gas info here.
For information about the different size bottles, check out this article