Rotting buildings are NZ's worst catastrophe, but the Govt is cashing in, says a study commissioned by the North Shore City Council.

Owners in two townhouse complexes have been backed in drawn-out leaky-building repair battles with their council, which lawyers say may affect rates.

A primary school with a long-running problem of rotten timber and faulty cladding is to be almost completely demolished.

The cost of the leaky building crisis has grown so "gianormous" that the Government has almost despaired of finding a solution.

The Government is moving on with its plans to help leaky home owners and says it is up to councils whether they participate or not.