While old drawings were probably sent to Germany, the contract called for them to design a new welded boiler to the same dimensions as the original boiler. This drawing was provided to a consultant working for the NSW Government. He approved the new drawing before the boiler was built. Presumably the drawing was seen by others before the boiler was built but nobody picked up the problem that it wouldn't fit.
Meiningen managed to build the boiler for "Tornado" and a boiler for a former GWR 2-6-2T (both to designs originally in feet and inches) and they are both still in use. There may be others. So why was 3801's boiler the only one that didn't fit?
I honestly don't know what the actual problem was but it didn't happen with any other boiler.
So the blame must be with the people who checked the drawing to which the boiler was built.
If an Australian company had done the same and the same discrepancy made in the drawings, the result would have been the same, if the people responsible for checking the drawing didn't pick up the difference.