Bowlin lost control at a right-hand bend, veering right then left and then leaving the road.
The summary says the van rolled "violently" over an open drain before smashing into a retaining post and brick feature wall, and coming to a stop upside-down.
Mr Gibbs, who was wearing a seatbelt, and Ms Walsh, who wasn't, died at the scene.
According to the results of a vehicle inspection, the van was operating normally before the crash, but its front tyres were worn on their outer edges and were "not of a warrantable condition".
Tests found Bowlin's blood alcohol reading was 113 milligrams, above the legal limit of 80mg.
He initially declined to comment to police.
When Bowlin was charged earlier this year, Ms Walsh's mother Ngaire Parker told the Herald on Sunday she welcomed the prosecution, but it wouldn't fill the gaping hole left in her life.
"We're still really upset about it all. Ashley was such a gentle little person, she was different to other teenagers.
"We've lost our beautiful girl, it doesn't bring comfort. We're glad that he's been charged but it doesn't bring comfort to Ashley's dad or me or her little sister that misses her every day."
