He said police were treating it as the boy found it, not theft.
When the police officer knocked on the family's door and explained, they handed it over "no problem at all", Mr Stewart said.
"In actual fact too, there's no criminality involved because it happened in Australia so it's obviously, from a legal point of view, outside our jurisdiction."
He said the officer who took on the job, Constable Andy Davis, had become a "guru" with these kinds of cases.
Constable Davis had received a similar job a couple of months ago, where he helped retrieve an iPhone from an address in Woodend, Canterbury that was actually lost at a concert in Auckland.
Mr Stewart said he himself was rather surprised by the amount of information Microsoft could tell them.
"They actually knew the whole path of that laptop."
He said Microsoft even worked out the computer had travelled on a boat - due to its slow speed across the Tasman.
"It's a good story with a good outcome and probably one of the better jobs you attend to when you're in the police," Mr Stewart said.