You should treat the SC member just like any other contractor who does work for you. They should provide a detailed quote.
Check your insurance policy to see what public liability is available for people working on the property. Talk to the insurance broker (or strata manager) about it.
Read Part 3 of the Strata Schemes Management Act 2015 (NSW). It says that the strata committee cannot make a decision which requires a special resolution to be passed. (Section 36 3(a)). An example of such a decision is when the external appearance of the common property is to be changed. If the proposed work involves changing the external appearance of the common property, the owners corporation must decide on it.
The owners corporation can override the strata committee if it disagrees with the Strata Committee. See section 36 paragraph (2) of the Act.
You can go to NCAT for an urgent hearing to stop work being done on common property.