The three people found dead at a Timaru address overnight were all siblings aged under 10.
Emergency services were called to the Queen St property just after 10pm, and on arrival they found three dead bodies. One other person was hospitalised.
The children were siblings - two of them twins aged three, the other seven, Det Insp Scott Anderson said on Friday afternoon.
A woman located at the property was taken to Timaru Hospital where she remains in a stable condition.
"Those involved had arrived in Timaru from South Africa last week," Anderson said, and that no one else was being sought in relation to the incident.
Area commander Insp Dave Gaskin said the incident "will be incredibly distressing for Timaru residents, particularly coming so soon after the town lost five of their young people in a crash", even if the family hadn't been in town very long.
"Police are working with our partner agencies to ensure that support is wrapped around those most directly affected by the tragic events of last night, and the wider community and we urge anyone who needs support to reach out."
Police said forensics expert were taking their time, still investigating the scene on Friday afternoon.
District commander Supt John Price said locals - including police officers - had seen "a lot of death" recently, referring to the August car accident which took the lives of five teenagers.
"My message to the people of Timaru is you live in a fantastic, safe place... we want to provide our reassurance that we are doing everything we can as police and with the wider partner agencies to ensure that Timaru will be safe and feel safe."
Many of the deceased's next of kin are based overseas, so more details about the family won't be released until they have been contacted.
'Noise like a deep moan'
Speaking to Newshub earlier on Friday, neighbours say they were watching television on Thursday night when they "heard sort of a noise like a deep moan".
Jade and Rob Whaley say they went outside to investigate, but couldn't see anything of interest. They went back inside and about five minutes later heard "a loud bang", prompting them to go back out.
"We couldn't see the spotlight had come on so we knew someone wasn't on the property," Rob says. "It sorta sounds liked a car door being slammed or a car being kicked. We couldn't hear anything so we went back inside again."
But while they were inside, Jade said they heard more moaning "going on and on", perhaps from the offender.
"They were moaning and making lots of noise like, 'What have I done?'"
The police arrived shortly afterwards.
The town's Mayor, Nigel Bowen, told Newshub locals were already struggling after the fatal crash in August which left five teenagers dead, and the COVID-19 lockdown.
"The young boys who passed earlier in the year, that was just absolutely devastating for us. I think the last funeral was just the day before everyone went into lockdown... now to have something like this, you can't say that it won't have an effect on the community. It's going to be really tough.
"It will have a ripple effect on families and friends who knew these victims and their families. It's going to be really tough in the times ahead. Bad luck comes in threes I believe, so hopefully that's it and we're good moving forward."
Police are expected to hold a press conference at some point later on Friday.
