"We want to remind anyone who may be assisting Walter to evade police that this is a criminal offence and we will be taking this type of behaviour very seriously. Walter should not be approached. If you know where he is, or he is sighted then please call 111 immediately," Mr Begbie said.
He said police have been working tirelessly on this investigation since October and have made a number of inquiries to locate Ngaau, however these have been unsuccessful.
Any information which can assist police can be passed on to Detective Sergeant Shane Pilmer on 021 191 5948. Alternatively, contact Crimestoppers anonymously on 0800 555 111.
One man has already been charged with the murder of Mr Harris and police say another man suspected of being involved is in custody on unrelated matters and will be spoken to about the murder.
Nicky Bryan Dodd, 41, from Raumanga, appeared in the Whangarei District Court on Tuesday charged with the two other men, including Ngaau, with the murder of Mr Harris.
It is nearly six months since 37-year-old Mr Harris, also known as John Boy, from Horeke, was driven by two men to the St John Ambulance with a gunshot to his chest on October 18, 2016.
He was loaded into an ambulance but died at the station about 6.40am. The death prompted a large police operation as the ambulance station on Western Hills Drive was cordoned off.