Mr Lepper and Central Otago District Council business development manager Jono Gadd, who were among the first at the accident scene, dived into the lake.
Their rescue attempt was thwarted by the depth and coldness of the water and they were unable to reach Aarush Macwan, who was buckled into his car seat in the van, 5m under the surface.
The Auckland boy, on holiday with his family, was later recovered from the vehicle by a diver, and pronounced dead at the scene.
The Toyota Hiace van rolled into the lake with five passengers inside, after being parked off the road, at Quartz Reef Point, 5km from Cromwell on the Tarras-Cromwell highway.
Four of the passengers, three women and an 11-year-old boy, escaped from the vehicle and were helped to shore.
Mr Lepper was shaken by the incident and said his thoughts were with the boy's family.
"It's a bloody tragedy. A vehicle simply parked on the side of the lake for a minute and the next thing, it's rolled into the water."
He was travelling to a meeting at Tarras that afternoon with Mr Gadd and district councillor Jeff Hill, when the father of the young boy flagged down their vehicle, frantically seeking their help, shortly after the accident.
"Jono and I stripped off a bit and dived into the water and tried to save the wee boy but it was absolutely freezing and the water was just too deep. You could see the van, but it was so far down we couldn't reach it. We had to give up ...
"There were a couple of young guys there helping too. I don't know who they were, but they dived in and were doing a good job. They got a woman passenger out and pulled her to the lake edge and then we helped lift her up over the rocks.
"I think she was the mother of the boy and to be honest, she wasn't looking so good at first, but then she coughed and spluttered up some water and seemed to come right."
Mr Hill said the men tried their hardest but the depth of water and the fact the van was upside down, made it a huge challenge.
"Everyone was pretty gutted, but they'd tried their best. To stay down there long enough you would've needed breathing apparatus."
Mr Hill said the boy's mother looked in bad shape when she was first pulled from the water, but later rallied.
"But once she came around a bit, you could see when realisation hit ... when she realised the wee fellow was still in the van."
Acting Central Otago police sub-area supervisor Mike Williams said the driver was out of the vehicle when it rolled into the lake.
The four passengers were treated at the Cromwell Medical Centre.
Victim Support volunteers were looking after those involved in the accident, he said.
Volunteer fire brigades from Tarras and Cromwell attended the scene and Cromwell fireman John Searle, who is also a diver, used breathing apparatus to recover the boy.
"CPR was attempted once he could be brought to shore, but he died at the scene," Sgt Williams said.
Mr Searle said the van was resting on a shelf, about 5m under the surface of the lake.
The young boy was still strapped into his car seat. Visibility was good in the lake, and the water was clear but very cold, he said.
