Detective Inspector Greg Murton earlier said the evidence is clear that it was a homicide.
However, he won't comment on the nature of the injuries or if a weapon was involved.
A post-mortem has been completed.
One resident on Tuesday recalled the murder of Marie Davis in April 2008. The 15-year-old's body was found in a similar part of the Waimakariri River 11 days after she went missing.
The resident said police searched the riverbank and the area around their house then. In that instance she and her husband had heard a car racing up the road late at night.
This time they had been watching the election debate and had not heard anything, she said.
"You just feel very sad for all the people who are touched by such a tragedy... It's just horrid to hear of these tragedies."
She said the area was usually pretty safe although she would not walk along the river on her own because criminals were known to grow cannabis there.
Another resident said he went for a walk down the road about 5pm on Monday and didn't notice anything unusual.
Police would like to hear from anybody who was near Harrs Rd and South Eyre Rd on Monday between 8pm and 10.30pm.
Anyone with information can phone Christchurch Police on (03) 363 7400 or Crimestoppers anonymously on 0800 555 111.
