Australia were bowled out for 198 in their first innings before Bangladesh piled up 426, taking a commanding 228-run lead.
The hosts were then dismissed for 284 in the second innings, leaving Bangladesh a target of just 57, which they chased down comfortably.
Cummins pinpointed Australia's first-innings collapse as the decisive moment, saying they never recovered after failing to put a competitive total on the board.
“Probably that day one. I thought our prep was spot on, no excuses. They played well. We had to find a way to bat longer and then we couldn't penetrate with the ball.”
The Australian captain acknowledged that Bangladesh had outplayed his side throughout the Test, praising their patience and discipline in conditions that had been expected to favour the hosts.
“They outplayed us in all facets, they were patient and disciplined.”
Australia's bowling attack, featuring Josh Hazlewood, Mitchell Starc, Pat Cummins and Nathan Lyon, struggled to make consistent inroads as Bangladesh's batters spent 138 overs at the crease in their first innings.
Cummins also highlighted Cameron Green's resistance in the second innings. Green scored a fighting 104, but lacked the support needed to build a match-saving partnership.
“Green's tempo showed how good he is, he needed a couple of us to hang in with him.”
The defeat leaves Australia with plenty to address ahead of the second Test, particularly with the bat.
Cummins said the team would assess their combinations and match-ups after reflecting on a performance in which Bangladesh controlled the contest from the opening day.
“Every time there's a game like this, you look at match-ups. We've just finished the game, so we'll think about it. We're pretty good at bouncing back.”
