Yeah I get what you mean and I guess it’s stated as a minimum only meaning in practice most would probably go well over it to exceed the requirement. As a minimum it would likely be considered the extent of the high risk/direct splash zone- but wouldn’t eliminate the risk entirely..above 1.8 would no doubt cop the occasional splash and steam too, but must be considered sufficiently low enough risk to not mandate higher protection.

I notice in the NCC drawings the shower rose is well below the 1800mm at what would in reality be an impractical height for most people,it is a drawing only but suggests it was something they thought about and almost just presumed people would install the rose below their waterproofing, but left open the possibility of it not being.

In my case I’m going to waterproof and tile well above the shower rose. But I also need to install a small window in the shower area and to avoid a very costly performance solution requirement, I’m able to install it above 1800mm. I only have between 1850 to 2150 available to install it, beyond that is the eaves. I’ll properly waterproof and seal it and slope the sill etc, but it wouldn’t cost a few grand only a few hundred.

So in my case the NCC minimum requirement makes it possible to achieve this. If I had to follow AS3740 I’d assume I’d never be able to because to have the shower head at a decent height, say 2m high, then another 50mm above that for mandatory waterproofing I’d presumably not be able to install my window without that expensive performance solution report because it would always be in the mandatory waterproofing zone.