Police area commander Inspector Simon Feltham said the 62-year-old man died before emergency services arrived at the home in Murphys Rd shortly before 3pm.
He had been doing work on the wall, which was part of a water feature.
Family members, who were "obviously very upset", were also home at the time.
"It's not a good situation at all," Feltham said.
The man's body was taken to Blenheim Hospital.
Police were still trying to contact extended family members.
The coroner had been notified and, although the death was not the result of a commercial activity, police had also contacted the Ministry of Business, Employment and Innovation as a matter of course, Feltham said.
Police would not release further information about the death or the man's name until all his family members had been notified.
