Two cats were also found dead in the front seat of the SUV.
"It's the whole picture. The adults, the children, the pets," McManus said.
"Saying it's not pretty — there's no words to describe that."
McManus said there was evidence "it was not an accident", and he did not suggest that anyone outside the family was involved.
He said the parents were in their 30s but did not disclose their names.
The family had military ties, he confirmed to reporters at a Thursday night briefing, but he would not say which branch.
McManus said the family moved into the house in January.
Where to get help:
• 0800 543 354 (0800 LIFELINE) or free text 4357 (HELP) (available 24/7)
• https://www.lifeline.org.nz/services/suicide-crisis-helpline
• YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757 or TEXT 4202
• NATIONAL ANXIETY 24 HR HELPLINE: 0800 269 4389
- AP