Good try Foolestroupe. it only took two mentions before the penny dropped.

Knowing my father had been in Darwin during WWII I asked him what unit he'd been in.
"None" he replied.
"Everyone in the army is in some unit or another," I protested.
"I was one of three surveyors attached to Colonel XXXX's personal staff. We'd go on surveying jobs." He went on to explain that they "borrowed" some American theodolites as they, unlike the British ones issued, gave 'erect' images; they were peeved they had to return them.
"So," I said. "If I were to ask you 'What important thing did you do in the war, Dad?' what would you say?"
He thought about this and then said "I laid out the sewerage system for Darwin after the bombing."

"Well," I replied, "It was probably the only thing in Darwin that survived Cyclone Tracy untouched."

Cheers, Rowan