A decision to make changes and/or additions to the Common Property (in NSW) can only be taken at a General Meeting of the Owners Corporation, where ≥75% of Owners present either personally or by proxy vote in favour of making the changes/additions; this is called a Special Resolution as opposed to an Ordinary Resolution where a simple majority applies; see S65A of the NSW Strata Schemes Management Act (1996).
It follows that neither your Executive Committee nor the Strata Manager can authorise or make the changes referred to in your post.
You should contact whoever you believe is behind the decision and advise them of the above, and advise that unless the work ceases immediately (please, and hopefully it will) you will immediately seek the intervention of the Consumer Trader and Tenancy Tribunal (Strata Division) by way of an Interim Order to restrain any further works.
So far as the design of and the contractor/s involved in the proposed works is concerned, that should all be tabled at the (Extraordinary) General Meeting – and by the way if the convening of that Meeting is delayed in any way, then 25% of Owners can request that be convened by way of a “requisition” to the Secretary of the Executive Committee (like a petition setting out what you want discussed and why) who must then act on that “without delay”.