ECan water quality team leader Graeme Clarke examines the bottom of the Opuha River at Skipton Bridge with a bathyscope on Wednesday.

Environment Canterbury (ECan) will look for further DDT granules in South Canterbury's Opuha River after removing granules containing the toxic pesticide.

Police confirmed on Thursday they were investigating the discovery, in August, of granules later determined to contain DDT and arsenic.

ECan had the granules as well as rocks and sediment removed from the Opuha River near Skipton Bridge, which is near Fairlie, on Wednesday.

JOHN BISSET/FAIRFAX NZ White granules identified as DDT have been dumped into the Opuha River near Skipton Bridge in South Canterbury.

READ MORE: DDT dumped in Opuha River

ECan's surface water science manager Dr Tim Davie said on Thursday the regional authority would take more sediment downstream from the bridge and consider a "riverbank survey" upstream and downstream.

Fish and water samples would be taken with Fish and Game and the Timaru District Council respectively, Davie said.

JOHN BISSET/FAIRFAX NZ A digger removes granules containing DDT and arsenic from the Opuha River in South Canterbury on Wednesday.

He said about two kilograms of DDT granules containing arsenic had been removed on Wednesday and no further granules of DDT were found near the bridge.

Opihi Catchment Environment Protection Society (OCEPS) chairman Allan Campbell said the "whole Skiptons deal" was "a smoke screen to hide the real facts". The former pharmacist insisted the granules "were formed in the river".

He claimed the group, one of whose members was present when the granules were first noticed, had video evidence of granules sitting under rocks and any suggestion "someone tossed the stuff in and covered it with rocks" was "a joke".

"These were formed in the river."

Davie said the pile of sediment "with white granules mixed into it" was "set behind some larger rocks".

"In the natural run of the river around Skipton Bridge, sediment does not deposit itself into small piles."

The Opuha dam upstream and the riverbed's largely stony composition meant any fine sediment deposited by the river spread over a wide area.

Davie said the high ratio of DDT to its break-down products suggested the granules had not been sitting in the environment for a long time.

He said one OCEPS video from May 2015 did not show the sediment or rocks in question.

"A later OCEPS video, from Sept 2015, shows some of the pile of rocks being lifted and there is clearly didymo beneath the rocks.

"The rocks must have been placed there recently or the didymo would have died."

South Canterbury District Health Board medical officer of health Dr Daniel Williams said it was "very concerning" someone would put even small quantities of pesticide in a riverbed.

However, there was "no evidence" the granules had contaminated the river.

Williams said the Opuha River fed into one of the sources for Timaru's town water supply and the council took the matter "very seriously".

Timaru District Council drainage and water manager Grant Hall said the council had not been taking water from the Opihi River, into which the Opuha River flows, for the last two weeks.

Opuha Water Limited acting chairman Milne Horne called OCEPS' latest claims "outlandish".

The dam company wanted "more responsibility to be shown by those continuing to make claims about Lake Opuha and the river that are not supported by credible science".

Horne said the company had received "many calls" from concerned people and would "fully support any and all investigations" of the alleged dumping.

OCEPS has claimed DDT was historically dumped in the bed of Lake Opuha. ECan chief executive Bill Bayfield said on Wednesday initial results from sediment elsewhere in the catchment showed "no evidence of a dump".

Campbell also said questions about why "South Canterbury's cancer rate" was "so high" needed to be asked.

Williams said "cancer rates in South Canterbury are similar to rates across the country".