Major renovations done without registration of a Bylaw. Who is responsible for remedial work?

I would like some help regarding who is responsible for fixing problems with water penetration resulting from major renovations of a penthouse in a small building of less than 10 units (in NSW – mod.).

Unfortunately there is no registered By-law of these works that were done over ten years ago and which I understand should have occurred. The owner, executive committee and strata managers at the time failed to perform than duty of establishing a By-law but seemingly approved the work.

The renovations comprised of extending the penthouse out and removing the wrap around balcony, building new walls to the exterior building perimeter extending the roof out and doing internal work to all areas. A previous owner did the work and the design faults have just been realized.

The work done is a dramatic change to the original structure. Who is now responsible for the ongoing maintenance? Should the original design be reinstated or the new owner required to take on the responsibility of fixing the problems or is the Owners Corp having to pay for the remediation or is there insurance that can be claimed for the past incompetence of the executive board or strata manager or is the building certifier at the time responsible?

The problem is – water leaks into the units below the penthouse and causes staining to the ceilings. The problem has recently been recognised as being due to faulty waterproofing membrane to the windows sills and/or with the tiling of the balcony and internal floors. The problem will be expensive to fix! The windows and tiling were part of the new work.

Other details are: Our strata has not adopted the LPI memorandum of 2011. The work is outside the 7 year builders warranty.

I would very much appreciate opinions on who is responsible for the costs of remediation and if we can correct the past and register a By-law for future possibilities.