Yep but I think it's only when there is reasonable doubt if GST is payable by the developer. For my OTP sales I know I will have to pay GST for sure so I wouldn't bother trying to sneak this one in, but maybe it's not a bad idea ;)
I think what's happened is the developer is probably not registered for GST so does not think he is liable for the GST upon sale (which may be true as well). It's just a clause to protect him if the ATO thinks otherwise about the GST question.