A member of the public saw Marsh lose control of his Nissan on the Tapanui-Waikoikoi Highway, wiping out a road sign in the process.
He drove off, so they called police.
Officers found him in the area an hour later.
Marsh told them he was driving back to his home in Heriot from Kaitangata, a nearly 100km journey.
He told police he had got lost during the trip and did not recall skidding off the road.
The damage to Marsh's vehicle told a different story.
When breathalysed, the defendant gave a reading of 1128mcg of alcohol per litre of breath — more than four and a-half times the limit.
Mr McCaskill described it as "quite high".
"That's a bit of an understatement," the judge responded.
Marsh's comments to police after the offence were unusual.
"In explanation the defendant stated he was driving because it was better than walking," a summary of facts said.
Mr McCaskill said that his client had been living in a caravan beside his parents' home but had an address available where he could serve a sentence of home detention.
Judge Macdonald agreed prison had not acted as a deterrent to Marsh's errant behaviour but, he said, it was the only way the public could be protected.
Marsh was jailed for nine months.
He was banned from driving for 28 days, which begins when he is released from prison.
The alcohol-interlock regime will follow.
