Detective Senior Sergeant Hamish Blackburn said the man was assaulted outside of his store in Hartham Place on Friday afternoon.
"Police are still working to piece together the full circumstances of what has occurred, but what we do know is that shortly before 2.50pm three men have gone to the store.
"One of the males has entered the store and the victim has followed him outside before being assaulted."
The shopkeeper had a serious head injury, and was now in intensive care in Wellington Hospital.
Blackburn said a team of police working through security footage from the area and working to identify who had been involved.
He called on the three men who had been at the store to get in touch with police.
He also asked for anyone who was in the area at the time or who had information about the incident, to call police on 105, and quote the file number 231007/8829.
Information could also be provided anonymously via Crime Stoppers on 0800 555 111.
