You can’t really work on the assumption that they won’t fix it properly (regardless of how likely you think that is). However, you are entitled to pursue this at Fair Trading, in the first instance, and at the obligatory mediation you can demand a written undertaking that they will sign the contract by a reasonable date, that this is not contingent on anyone else’s roof being fixed and that it must be done by a licensed professional with the relevant warranties and insurances. If they refuse on any of those points, you proceed to a Section 232(2) at NCAT.
You could also seek a Section 238 order to have the obstructive members of the committee removed for failure to manage the building according to the Act, but that’s a longer shot.