Southland farmer Warren MacPherson has told Winston Peters at a public meeting that Environment Southland failed to report a 23,000 litre oil spill into Sharks Tooth Creek, Spring Hills, Southland, in late October, 2015.

Environment Southland "covered up" a massive oil spill into a Southland waterway following a truck accident, a public meeting in Invercargill heard.

Farmer Warren MacPherson made the claim during the public meeting on Monday morning.

MacPherson, who is himself being prosecuted by Environment Southland, claimed 23,700 litres of oil was spilt into the Sharks Tooth Creek, Springhills, on October 30, 2015.

NICOLE JOHNSTONE/STUFF Warren MacPherson claims oil from the spill ended up in the Makarewa River.

A truck carrying the oil had crashed and tipped over, he said. The oil had flowed into the Makarewa River, MacPherson said.

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Environment Southland, in a long statement on Monday afternoon, confirmed the oil spill had occurred, but said it did not routinely publicly notify incidents and it had been "assumed" the spill would probably be reported to the media via the emergency services.

When an Environment Southland compliance officer arrived at the site after about 40 minutes, most of the oil had escaped the scene, Environment Southland senior staffer Vin Smith said.

A small tributary was dammed with dirt and baleage, which had captured some of the oil and, soon after, the remaining oil was removed using several sucker trucks.

The truck company took full responsibility for the cleanup, and the company's estimated cleanup costs were more than $250,000, Smith said.

"Southern Transport spent over 1000 hours of Southern Transport's and contractors' time cleaning up the initial spill and subsequent releases from captured oil from the plants and banks of the river," Smith said.

"The company ended up using over 120 oil absorbent booms, 600 oil absorbent pads, diggers, tankers and vacuum trucks in its response."

MacPherson earlier told NZ First leader Winston Peters at the packed public meeting: "I am prepared to state here and now there was a monstrous oil spill in October 2015 where 23,700 litres of highly toxic transformer oil from the Roxburgh hydro dam was being transported to Invercargill ... 23,700 litres went down the Sharks Tooth Creek and eventually into the Makarewa River."

MacPherson, the chairman of the Makarewa River liaison committee, complained that Environment Southland had not done enough."All of that oil went down, right out to sea. No ratepayers were notified."

No-one had been notified the oil was potentially highly toxic to animal health, so people taking water out of the river for stock or for human consumption may have ran the risk of illness, he said.

MacPherson said he worked with Environment Southland after the spill and they tried to work out a system to put measures in place to contain any other spills in future.

After the meeting, MacPherson said he was being prosecuted for a "minor spill" himself, understood to be farming related, and he believed there were different sets of rules for some, compared to others.

Smith confirmed the Southern Transport Co Ltd truck that crashed was carrying about 23,700 litres of transformer oil.

The "skin" of the truck was compromised and the oil escaped into Sharks Tooth Creek, a tributary of the Makarewa River, he confirmed.

The police and fire service were called and were on the scene in very short time frames, and an Environment Southland compliance officer was 40 minutes from the scene.

Environment Southland staff had monitored Southern Transport cleanup efforts and had regular meetings with the manager of the company.

The incident was investigated and assessed the same as other incidents, and it was determined Southern Transport had a strong defence to a prosecution so it was not taken to court.

Environment Southland was contacted by MacPherson requesting a meeting some months later, which he and some other community members attended.

There was also a debriefing that included the contractors, Southern Transport and the fire service, Smith said.

"Many lessons were learnt from this incident and, as a result, Environment Southland has stepped up its level of specialist training and equipment.

"Additionally, changes have been made to our response protocols with regard to incident escalation and communication."

Environment Southland did not publicly name people or companies it was investigating or prosecuting, he said.

Environment Southland chairman Nicol Horrell, when asked why the public wasn't informed of the spill, said lessons had been learnt and "we could have done better".

If it happened today, the public would be informed, he said.

He did not believe there was any evidence that human health had been put at risk as a result of the spill.