The thing to look for is was this work authorized by the EC or OC via a valid motion of the EC or OC. Must be in the minutes of any EGM, AGM or EC meeting. The Chairman has NO special powers to authorize any spending (unless he has been granted this power at a EC or AGM again check minutes).
If this was authorized at a AGM or EGM then you are liable to pay the bill.
If it was authorized at a EC meeting you still might be liable it just depends on the spending limits of the EC at the time (I cannot find the default figure in the Strata Act but is is normally about No of lots x $200), but this can be changed by a motion at a AGM or EGM. The EC themselves cannot change this.
In a few instances EC members can be authorized to spend up to a certain figure, you need to ensure the Chairman didn’t have this authority.
This all assumes you don’t use the designs, if you do you are liable for the cost whether it was approved or not.