Horizons Regional Council and Fonterra have also been involved in the aftermath of the incident. Horizons communications officer Cara O'Neill confirmed the council's environmental protection team was investigating whether One Plan rules had been breached.
Details of the incident have been passed to the Civil Aviation Authority, the lead agency under the Hazardous Substances New Organisms Act for widespread aerial application.
Fonterra visited the dairy farmer who leases the sprayed land and technical and assurance general manager Andy Goodwin said assessors had determined two blocks had been sprayed within a few weeks.
On the first occasion hill country had been sprayed and Fonterra was satisfied the dairy farmer had observed the recommended withholding period before returning cows to that pasture.
The land sprayed on November 8, said Mr Goodwin, was locked up for silage making and Fonterra believed those paddocks had not been grazed since. He said Fonterra could say there was no danger of contaminated milk being collected from the property. The farmer's dairy herd had also been assessed to ensure the animals were in good health.
Fonterra took any complaints of spray drift and its consequences seriously and there were "concerns" from investigating parties that neighbours had not been advised of what was being applied.
An inquiry to the helicopter company by a neighbour after the aerial spraying of the hill-country block is reported to have brought the response that a "slurry fertiliser" was being applied.
There was said to be no mention or acknowledgment that herbicide was being sprayed.
In the wake of the November 8 incident further questions were asked of the company regarding the spraying with a view to determining whether Thistle Killem was, in fact, applied but an answerphone message went unanswered.
Thistle Killem is applied to kill thistles and other unwanted weeds such as buttercup.
The ill farmer's sister, who travelled to Eketahuna to help with farm work while he was incapacitated, said MidCentral Health protection officers had been alerted after the farmer had initially sought help from the 0800 POISON line.
"My brother was not able to carry out his usual farm work and has concerns over the welfare of his animals. Working dogs have shown signs of poisoning and are now improving, but concern is held over long-term damage that may have occurred."
As her brother had slowly regained his ability to think more coherently about his immediate medical concerns and his stock, he had contacted family and others living in the area to create further awareness of the issue and its seriousness.
The woman said questions needing to be answered by investigators included whether all farmers potentially affected had been alerted to the fact livestock should be checked for a reaction to the un-notified drop.
She and her family also wanted to know what impact the poison could have on waterways and off-roof water supplies.
ACC was funding half the cost of the affected farmer's medical expenses, which had involved doctor visits and tests, but there was no guarantee there would not be "life-long expenses".
"This is a clear case of negligence and noncompliance with regulations in the use of dangerous chemicals.
"The use of this product needs to be restricted and monitored rigidly to protect the lives and incomes of environmentally conscious and law-abiding New Zealanders," she said.
