Hi Frazer,
In Victoria, the boundary of a lot will be interior, median or exterior face. The boundaries between floors is most likely either interior or median and in either case the waterproof membrane is most likely a lot owner responsibility. This has been determined by VCAT a number of times but will depend on the interpretation of your plan. In addition, it is more likely that it was the failure of the membrane that caused the damage of the water leaking that is the subject of the claim rather than shower base itself.
Membranes fail overtime and is not considered an accident; hence it is not covered by insurance.
With respect to the shower base itself, you could ask the insurer why the shower base itself was not covered.