The road also remains closed.
"Police are still working to ascertain the cause of the woman's death, and a post-mortem is scheduled to take place tomorrow," Mr Sweeney said.
"However, the current circumstances lead us to believe this is a homicide investigation."
Police are still working to complete a formal identification of the victim.
"Until this has been done, and her family has been advised, police will not be in a position to release a name."
The manager of the dairy farm where the woman's body was found was told by police the woman had been "missing for a little while".
The farmer, who only wanted to be known as Jori, knew about the fires in the area last night.
But it wasn't until he got up about midnight that he saw police outside his property.
"I went to have a look and they told me there was a body there, a lady who had been missing for a little while," he said.
"[The body] was right by the boundary fence, between the farm and the road.
"They said they were looking for two occupants of a 4WD who had been in the Rakaia river."
Local dairy farmer Michael Norton said thieves stole two 20-litre containers of petrol from his farm on Main Rakaia Rd on Saturday night.
He said they smashed the lock off a shipping container and targeted the petrol, leaving behind tools, a water blaster and pump, of "more value than petrol".
Mr Norton noticed the break-in yesterday afternoon and reported it to police today.
"I thought with all these fires and the body, and having petrol stolen like that, it seemed like a strange thing," he said.
"So I put two and two together, thinking there could be a link there. Obviously we don't know that for sure, but it made me a bit nervous."
Earlier yesterday afternoon, a series of suspicious fires had been lit around 4pm in the Selwyn riverbed near State Highway 1, just north of Dunsandel.
It sparked a large emergency response, including four fire engines, 10 water tankers and three helicopters.
The Fire Service was first called to a fire at Selwyn river at 4.44pm.
When they arrived, they found two fires, one on either side.
"Obviously the guys were running around the river, lighting them," said Rakaia Volunteer Fire Brigade station officer Tyrone Burrowes, who was sure the fires had been started deliberately.
Two crews fought the fires, with assistance from a helicopter.
Later, at 7.37pm they were called to another scrub fire on Main Rakaia Rd.
The nearest unit was Kirwee Volunteer Fire Brigade.
As they were on their way, the member of the public, a "passer-by", rang back to give them a "heads up" that there was a body there.
Mr Burrowes wouldn't say whether the woman's body had been burned or was on fire.
He confirmed that Kirwee Volunteer Fire Brigade was first on the scene.
"They extinguished the grass fire and protected the scene for police," he said.
At 8.30pm, there were reports of yet another suspicious fire, this time at Overdale, south of Rakaia township.
A Rakaia tanker and Dunsandel fire crew responded.
Mr Burrowes believed that all of the fires were suspicious.
Police were yesterday asking for sightings of a 4WD vehicle that was seen in the Selwyn River in the vicinity of the fires when they were first lit.
Mr Sweeney referred queries to the police media headquarters in Wellington.
"We have released as much information as we have available at this stage," a media spokeswoman said at 10.30am today.
Canterbury police and forensic specialists are carrying out a detailed examination of the scene.
Drivers wanting to access a cordoned off section of Main Rakaia Rd are being told by the police scene guard it may be closed off for several days.
The road is closed between State Highway One and Old South Rd.
It's a stretch of road that looks very similar to any other in rural New Zealand, with a milking shed and irrigators visible in the cordoned off area.
The only thing to suggest it's far from normal is a bright fluorescent light, 300m from the cordon's edge which has been lighting the scene overnight.
Anybody with any information in regard to suspicious vehicles or people in the area are asked to contact Christchurch Central Police Station on 03 363 7400 or Crimestoppers on 0800 555 111.
Locals are upset by the grisly find - the second such disturbing body discovery in the rural Selwyn district in the last three weeks.
The badly bashed body of Marcus Luke Tucker was found burned, dumped, and wrapped in some material on Drain Rd, Doyleston by Lake Ellesmere on Anzac Day.
Peter John Carroll, 51, of Bishopdale, Christchurch has been charged with his murder. Three others have also been charged in connection with the case. All four are now before the courts.
