He returned on Sunday to take a photo, which he posted on a Kaitaia Facebook page. It quickly a host of outraged comments.
Anahera Herbert-Graves, chief executive of Te Runanga-a-iwi o Ngati Kahu, saw the post and alerted the Northland Regional Council's Kaitaia manager, Peter Wiessing, who said the dumping wasn't just ignorant and disrespectful, it was also criminal.
The hoggets had been stolen from the Rangiputa Station.
The offal could have degraded water quality in the ecologically sensitive lake, and put human health at risk, Mr Wiessing said.
Council and Department of Conservation staff responded quickly, but were beaten to the job of cleaning up by Rangiputa Station worker Boyce Lawrence.
"Thank God for local heroes like Boyce ... he's a man of few words, but he gets down and does what needs to be done," Mrs Herbert-Graves said. She said it beggared belief that anyone would want to defile the lake.
"I can't even start condemning the people responsible, because I won't be able to stop," she said.
Rangiputa Station manager Scott Hussey said sheep rustling was an ongoing problem, but was especially bad over the summer holidays and long weekends.
A large number of ewes had been stolen, as evidenced by pools of blood in a paddock where they had been shot and dragged to a truck, the Monday before.
A week earlier a group of children aged 10-15 had been caught in the act on a neighbouring farm.
The sheep's throats had been cut but the young rustlers had been nabbed before they could steal the meat.
The lake was used by many of his staff and their children, and they were angry.
"You wouldn't want them to catch you doing it," Mr Hussey said.
Mr Wiessing said there were similar problems around Northland, with people dumping fish waste in swimming areas.
Children ended up with spikes in their feet, and the offal attracted sharks and stingrays.
He urged people to dispose of animal waste by burying it at least 50m from water.
Some people mistakenly believed eels would clean up offal dumped in fresh water, but if there was too much of it it would rot.
Discharging animal waste into water was an offence.