Police investigating the murder of John Isaac say he was subject to a "violent attack before his body was found in the Waikato River".

The investigation, dubbed Operation Angelo, was launched when the 53-year-old's body was found in the river by a person passing by in a boat around 6.30pm, on Thursday, January 18.

Detective Inspector Darrell Harpur says they now believe John was the subject of a violent attack that afternoon.

"Mr Isaac’s movements leading up to his death are still being pieced together, and we would like to speak to anyone who may have seen or interacted with him in the days prior to last Thursday.

"He was last seen around 3pm on Wednesday, January 17, wearing black cargo pants and a green t-shirt with a print of a gasmask while shopping on Avalon Drive, Nawton.

"Our investigation is focusing on the Waikato River and surrounding areas, and the public can expect to see a continued Police presence in the area as enquiries progress.

"Mr Isaac was a son, a father, grandfather, and a much-loved friend. The circumstances around his death have had a severe effect on his whanau. They are being supported by NZ Police and Victim Support.

"Somebody out there knows who is responsible for this, and exactly what happened. We urge them to come forward and tell Police what they know.

"This is a tragic end to a life that should not have been taken. We have a dedicated team working tirelessly to get answers on what happened to Mr Isaac, and to hold whoever is responsible, accountable for their actions.

"This all occurred in broad daylight, and we believe there are people out there who have information that will greatly assist our investigations.

While we do not believe there is any ongoing threat to the public following this incident, Police will continue to have a visible presence around the area in the coming days. If you see anything that looks suspicious or have any concerns, call Police on 111.

"We want to hear from anybody who was around the Waikato River area in the afternoon of January 18 and saw anything untoward, especially around any of the bridges between Hamilton and Ngāruawāhia.

"We believe some people will also have photos or videos from recreational activity in the river and the surrounding areas, we ask that you reach out to Police to provide anything that can assist."

Anyone with information is asked to call 105, or make a report online at 105.police.govt.nz, clicking “Update Report”. Please reference the file number 240119/8775.

If you have footage, this can be provided to Police via the following link - https://angelo.nc3.govt.nz/ (link is external)

Information can also be provided anonymously to CrimeStoppers by calling 0800 555 111.