Take a look at this site and download LPI Circular 2011/08
As to your problem my feeling is that the shower tray is lot property and so your responsibility to maintain and any damage resulting from it leaking is yours to repair.
If the shower floor is leaking how do you know it’s leaking is it showing up as a leak downstairs if so it’s a Strata problem, if it is just seeping below the new tiles it’s your problem.
One problem could be how old is your unit because the waterproof requirements are those relevant when the building was built and maybe a waterproof membrane was not required in those days beneath the bathroom floor (just guessing as I don’t really know).