The rate of homicides in the country has almost doubled over the past three weeks, with police saying they have started eight separate investigations since December 16.

The police's latest annual homicide report shows that, between 2007 and 2020, New Zealand has averaged one murder or manslaughter every five days, not counting the Christchurch mosque shootings.

Yet over the past 21 days there have been eight homicide probes.

They include the fatal shooting of a Lower Hutt man at a birthday party at his home in mid-December, a Manurewa teenager killed on New Year's Day and that of a 38-year-old male found dead in a Christchurch street on Thursday.

They are also looking into two other deaths in Auckland, two in Hamilton and one in Hastings.

The Police Association Te Aka Hāpai says the dramatic increase in homicides over Christmas and New Year is unheard of.

Vice president Steve Watt says it's something he has not seen in his 23-year career.

He says stress may play a part in some of the cases.

"The pressures that comes on families around this holiday period certainly plays a factor in the increase in violence, and the jobs that we're attending.

"It culminates over a whole year of pressure, and tends to have that explosive element to it at this time of year."

Senior Sergeant Watt says the spike in homicides is also putting extra pressure on staff, with the holiday period already a busy time.

"With the amount of homicides we have been having, alongside our business-as-usual jobs that we have to deal with, there's an increase in demand from the public... and that just adds to the relentless pressure that the members are under."

A police spokesperson says it's difficult to attribute patterns or trends in homicides - "even when it may appear there has been a 'spike' or 'cluster'" - because each case "has unique circumstances".

The spokesperson adds: "Each of these deaths is tragic and will have far-reaching and devastating impacts for the victims' families and loved ones.

"These deaths are all the subject of extensive investigations, or are already before the courts."

Homicides since December 16:

• 16 December - a man was shot dead at a birthday party at Lower Hutt
• 18 December - A security guard, 25, is found dead in West Auckland park, Massey
• 20 December - A 24-year-old man died after an alleged assault in Hastings
• 21 December - An inmate, 47, at Auckland's Wiri prison died after an assault in the facility's kitchen
• 21 December - A 62-year-old man is found dead after what police call an 'altercation' in Hamilton East
• 30 December - A 24-year-old man is found dead in driveway in a Hamilton suburb
• 1 January - A 19-year-old teenager is shot in a vehicle in South Auckland early in the morning on New Year's Day
• 4 January - A 38-year-old man is found dead on a Christchurch road

-Rachel Helyer Donaldson/RNZ.