The rifle and its case have been recovered, but the service-issue notebooks are still missing.
They are used by officers to record details of on-duty interactions with the public.
Police say they are notifying between 350 and 400 people about a possible breach of their personal information such as birth dates, home addresses and driver's licence numbers.
The privacy commissioner's office also been contacted.
The officer involved is on paid leave while the investigation continues.Suggest a correction