About 80 protesters marched up Stuart St, in Dunedin, and tried to enter the office of accountant Murray Valentine, leaseholder of Simons Pass, near Twizel.
They were demanding Mr Valentine stop the intensive dairy conversion of 4500ha of the station.
The protesters hoped to present him with a petition but found the door to his office locked.
Mr Valentine said while he disputed many of the group's claims, they had the right to protest.
"I take my hat off to them this time. They were very well behaved and worked within the law.
"They didn't do anything I could object to and they've expressed their views."
The conversion had taken at least 15 years and had already gone though a long list of resource consents and Environment Court action, so it was uneconomical to stop it now.Milking on the farm had already started; new milking sheds would be built by September and irrigation would start in October.
He disputed the protesters' claim up to 15,000 cows would be farmed on the land, as only a maximum of 5000 were now planned.
There were procedures and monitoring in place at the farm to ensure there was no impact on the surrounding environment, including nearby Lake Pukaki, he said.
But one of the protesters, University of Otago emeritus professor Sir Alan Mark, said the Mackenzie Basin was at an environmental tipping point and the dairy conversion would push it over the edge.
"Dairy has a huge impact on the native flora and fauna in the past 10 or 15 years and this conversion is going to tip it over the edge," Prof Mark said.
Successive governments were to blame for encouraging the conversion of land in the basin, which had led to the destruction of large areas of significant native habitats, he said.
