Hot water cylinders are usually stolen from properties which are vacant for quite a while or seem to be vacant for a long time.
You can try to prevent your hot water cylinder by doing the following:
1) Install an alarm on your property.
2) Install a fence and gate.
3) Empty the letterbox as often as possible.
4) Mow the lawn every fortnight.
5) Ask a trustworthy neighbour to keep an eye on the property.
The photos above show parts of a house after the hot water cylinder is stolen.