In addition to the reward, the largest offered in a New Zealand case, police would consider giving anyone who came forward immunity from prosecution as long as they were not the main offender.
"The accomplice is the person we are directly aiming at. They are the people that would potentially be the recipients of the reward."
The reward is three times higher than the highest previous reward offered by police in the Operation Park rape cases in Auckland in the 1990s.
Two thirds of it has been put up by British collector and medals expert Lord Michael Ashcroft and the difference by an anonymous New Zealand businessman.
Mr Broad said the information given would have to lead to a conviction for the reward to be paid.
He said the reward was for up to $300,000 and he would ultimately determine how much of that any person coming forward would receive.
Detective Senior Sergeant Chris Bensemann said 12 officers, based in Taumarunui, were still working on the investigation.
Police were still in a "suspect phase" and were keeping an open mind on the suspect, their motivation and whether the medals were still in New Zealand.
Police were still wanting to hear from anyone who knew anything about a light-coloured 1980s station wagon seen outside the museum on the night of the heist.
Mr Bensemann refused to reveal the details of the closed circuit camera footage from the museum on the night of the theft, saying it was "operationally sensitive".
Chief of Army Major General Lou Gardiner urged anyone with any information to come forward.
The medals are considered to be worth several millions of dollars. Victoria Crosses are valued at about $500,000. Two rare George Crosses, an Albert Medal and a Distinguished Conduct Medal were among the others taken.
Among the stolen VCs were those of World War 2 soldier Captain Charles Upham, the only fighting man to ever win two VCs, the highest decoration for gallantry in the British Commonwealth.
Capt Upham's medals have been described as the "holy grail" by Lord Ashcroft.
- NZPA
