DDT and arsenic sat beside a South Island river bank for more than a month because test results took longer than expected, says the Canterbury Regional Council.
The white granules were first noticed near Skiptons Bridge in the Opuha River in August, but were not picked up by a digger until yesterday.
Timaru is still not taking drinking water from the catchment, with the District Council, and Environment Canterbury (ECAN) doing a final round of testing now.
Tests showed the granules in a kilo of dumped dirt contained high levels of arsenic and DDT, a pesticide banned since the 1960s.
Opihi Catchment Environment Protection Society spokesperson Allan Campbell claimed the Regional Council and ECAN were slow to test them.
In early October, weeks after they were first found, ECAN said its initial testing showed no sign of arsenic.
ECAN chief executive Bill Bayfield said today that further testing - which showed up DDT and arsenic - took longer than expected.
"Everyone looked at the flakes and thought what are they, tests showed they were DDT and arsenic. We got the results a week ago, and when we got the advice from an environment scientist that we should clean it up, we did so the same afternoon."
Mr Bayfield credited Nick Wall from the Opihi Catchment Environment Protection Society for being the first to raise the red flag about the contamination.
But he said there was no need to remove the granules when they were first noticed in August.
"Officers assessed that the prills weren't going anywhere and also given it was potentially a criminal site so we wanted to handle it properly and that takes time."
Timaru usually takes at least 40 percent of its water from the Opihi river - which connects to the Opuha - but has stopped taking any for the past several weeks.
District council services manager Ashley Harper said further testing was being undertaken, with the results due next week.
David Williams runs a sheep farm on the banks of the Opuha Dam and said allegation that the DDT granules or prills may have been carried down from the Opuha lake did not stack up.
"The prills are up on the bank and the flow of the river is the highest it has been in months, so if they prills were found above the water level they didn't come down the river, they've been placed on the bank."
Mr Williams was non-plussed at the ongoing claims that barrels of DDT are buried under the lake, as ECAN had done numerous tests over the years, which had all come back clear.
"We have to remember now that everyone on the other side of the world is reading what is happening in New Zealand and they are our biggest export market, this has far bigger implications than anyone realises."
Police are looking into the dumping on the river.
Detective Senior Sergeant Richard Quested said it was complex because it was an isolated spot.
"Our investigation is in a very early stage, so we ask any members of the public with any information to give us a call at the Timaru police station."
ECAN will be carrying out, water, sediment and fish testing around Skipton's bridge and downstream to see if there was any arsenic and DDT contamination.