Pete – I hope you wrote down the name of the Fair Trading employee that you spoke with, because you’ve stumbled upon the one who actually understands Strata 
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Regarding the responsibility for payment, as it’s the current Owner’s bathroom and they and subsequent Owners of the Lot will be responsible for the on-going maintenance and repair of that entire bathroom post its renovation, I’d suggest that the current Owner obtains the quotes, has each itemised to identify as closely as is practicable the two (2) cost components of the work involved (i.e. the O/C and the Owner), and then submits those to the Owners Corporation (O/C) with a indication of their preferred quote / contractor.
In that way, unless its vehemently against the Owner’s preferred quote /contractor, the O/C can simply make a payment for its cost component of the work directly to the Owner, who will then be responsible for commissioning the work and paying the entirety of what will then be their contractor’s invoice.
As for the required Statement of Compliance under AS3740, if the bath needs to be removed in any of the circumstances such as I earlier described (and it appears that’s the case) and the Owner wants to pay for and install a new one, if any contractor then refuses to warrant the entire job, the Owner should find one who will.
One final word form me. I don’t know if you’re representing the O/C or if you’re the Owner of the bathroom, but in either case don’t get too hung-up on itemising the respective cost components down to every tap washer and bath plug, but rather stick to the pareto items lest you frighten-off the contractors.