"Further charges may be possible," police said in a release.
The incident has rocked the Matamata community - Matamata Piako District Council Mayor Jan Barnes labelling it "very, very sad".
"So tragic, there is no winners in any of this," Barnes said.
Waikato police Detective Senior Sergeant Stephen Ambler said emergency services were called to a Matamata property at 1pm today.
It is understood the property was rented.
Jamie Kennedy, a farm manager of a neighbouring property, said he just had arrived back to find emergency services had cordoned off the property.
There were three different houses down the next door driveway, he said.
"I just came back from golf and thought, 'What the hell's going on down there'."
He had no details about the tragedy that had occurred.
Blandford Lodge stud director Graham Bax told Stuff the Westpac Rescue Helicopter had arrived at the scene, which was directly behind his property.
"There is a bit of activity going on ... I'm about 200m away," Bax told Stuff. "I went out to see if the horses were okay, because I was a bit worried about the noise of the helicopter frightening them."
An ambulance and police vehicles were also in attendance.
A woman posted on Facebook: "Heard the sirens before. Very sad, sending my love to the women [sic] who has passed and that poor little baby xxx."
Another neighbour believed a brother and sister lived at the property.
"We have seen her pushing a pushchair with the baby in it down the driveway," they told Stuff.
"They haven't been living there very long ... a couple of months."
The Herald understands the chopper responded to an incident at a house on State Highway 27, Matamata.
