I think the strata firm is bluffing – they would, wouldn’t they?
There is a basic flaw to their argument – how could the wider scope be impliedly authorised, when it would require a resolution of the OC? That is, I presume the fees fell within section 80D, so you couldn’t go any further without a resolution of the owners.
You know the strata manager signed a costs agreement that was contrary to the OC resolution, the mistake was theirs. As a first step I suggest you contact Fair Trading.