What can a lot owner do when the council of owners won't action water ingress repairs?

I can only take your word that the water leak is coming from a common wall.

Usually experts are brought in to determine the exact location of the water source.

Once that is established and if it is coming from a common property area, then the strata company have a statutory obligation under section 91 to repair and maintain the common property.

91. General duty

1. A strata company must —
   1. [(a) deleted]
   2. control and manage the common property for the benefit of all the owners of lots; and

   3. keep in good and serviceable repair, properly maintain and, if necessary, renew and replace —

      1. the common property, including the fittings, fixtures and lifts used in connection with the common property; and
      2. any personal property owned by the strata company,

      and to do so whether damage or deterioration arises from fair wear and tear, inherent defect or any other cause.

The strata company may have financial constraints in that they don't have sufficient funds to cover the quotes received; in which case they would have to convene an Extraordinary General Meeting (EGM) to raise the funds for expenditure.

Any further lack of action would place your insurance policy at risk from knowing about a defect and not reporting it to the insurer.

An application may need to be made to the SAT – State Administrative Tribunal.