In doing so, Treviso secured their tenth win of the season - the first time they have reached double figures - but Scarlets still squeezed into the play-offs thanks to Leinster's win over Ospreys and will travel to Ravenhill to face Ulster.
And Rees, who will join the Cardiff Blues next season, conceded Treviso were well worth their victory.
"We were very poor, the way we had performed in the last couple of weeks we were very confident," he said.
"Credit to Treviso they played for each other and we just didn't perform and I think their physicality and their forward pack dominated us.
"We definitely didn't underestimate them, we spoke about it all week, they're coming here with nothing to play for and they capitalised on our errors and it cost us the game.
"It's been great, I've got some great memories and I think the performance here wasn't a great way to end but we can look forward to Ulster next week."
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