A man and a woman were found dead from stab wounds at a house in central Auckland suburb Epsom about 11.30am in an incident a senior police officer has described as "a tragedy".
Two other people were injured - one critically, and one superficially.
The two dead people and the person in hospital were known to each other.
Detective Inspector Scott Beard said they were not looking for anyone else in relation to the double killing.
"This is a tragedy," Beard said.
"It would appear they were known to each other, but I can't give any more detail."
A post mortem would take place on Saturday and Beard said police were yet to formally identify the two people who died.
"Upon police arrival, two people were located with critical injuries," Beard said.
"First aid was immediately provided, however both have sadly died at the scene. A third person found at the property has been taken to hospital with critical injuries."
A fourth person who was hurt had only sustained superficial injuries, Beard said, and he was able to speak to police.
Paramedics arrived at the scene first and called police to the house.
Police reassured the public they were not looking for anyone outside of the four people involved in the incident.
In a later statement, Beard said that the people involved were known to each other and the incident was not a random attack.
Part of the police investigation would be providing victim support to the families of the deceased, Beard said.
Investigators would now door-knock the area to seek more information but also to reassure neighbours, he said.
The incident happened at a house on The Drive - a major thoroughfare through Epsom.
Police said there would be a further update on the incident later today.
Neighbour: 'I'm really shocked'
Neighbour Reva Kaushal was home with her husband and two daughters when the incident began but had heard nothing. She only realised something was happening when police cars began arriving.
In three years living in the area she had never heard any trouble.
"I'm really shocked ... and I'm a bit scared."
Her husband said he saw "an old man with a bandage on his hand".
Another neighbour, Audrey Zhao, said she was home with her children, aged 6 and 9, this morning and had not heard any indication of a problem.
She was relieved to have her children with her.
"Today is a teacher-only day, which is so lucky. I'm definitely nervous because it's so close to us.
"The kids aren't scared, but as an adult I'm glad we have security at our home."
Emergency services were called to the property about 11.30am.
Two people died at the scene. Another critically injured patient was transported to hospital.
Epsom MP David Seymour expressed shock and sadness at the news.
"This is shocking and tragic for our community. It is important that the police are able to do their job and the privacy of people connected with the incident is respected," Seymour said.
"There will no doubt be a lot of questions, and our community will want and deserve to know how and why this happened. In the meantime we need to respect the appropriate processes.
"My thoughts are with those lost and those who have lost in this tragic incident."
Police are making a number of initial enquiries to establish what exactly has taken place.
St John confirmed paramedics treated two people - one in a critical condition and another in a minor condition - and took them both to Auckland City Hospital.