Wellington police commander Inspector Jevon McSkimming said it was far too early to say who sent the envelopes.
"However, we can confirm that the powder was harmless baking powder. An investigation has been launched and will be looking into who might have sent the envelopes. It is too early to stay whether the envelopes were connected.''
Police are interviewing the staff members involved.
Neither Mr Brownlee or Mr Dunne are believed to have been in their offices at the time the envelopes were opened.
Ken Gurney, who was working in the electoral office when the envelope was delivered, said Mr Dunne's secretary Shirley Simcock opened the package.
He and the other workers were evacuated and were about to be given chemical showers when the all-clear on the substance was given.
"It's better to be safe than sorry and it's good to see all the emergency services so punctual and on the spot.''