Jacek Gryzbowski had been missing for a week after going for a solitary hike in Rimutaka Forest Park, the New Zealand Herald reported.
Senior Police Sgt. Murray Johnston said the body had not been formally identified.
"We have notified the Polish ambassador and the coroner that, sadly, we have found a body," Johnston said. "We've made it clear that we can't yet determine that it is Jacek because the body is trapped in log debris in the stream."
The search has been hampered by bad weather, including high winds that prevented the use of helicopters. Police said if the weather remains bad Sunday, the search team may have to bring the body out on a stretcher, a long and difficult process.