The Brumbies have unravelled after building a promising lead, falling to the Blues 38-10 in Auckland to complete a tough month for the beleaguered Super Rugby outfit.

Hooker Connal McInerney scored from a Brumbies maul to turn a period of dominance into a 10-7 lead with five minutes to play in the first half at Eden Park.

But they went to the break down 17-10, a penalty after a sloppy restart followed by a weaving try to Finlay Christie that exposed some flaky defence close to the line.

The Blues didn't give Dan McKellar's men a sniff in the second half, AJ Lam's try with 20 minutes to play creating a 31-10 lead before Folau Fainga'a was yellow-carded after continuous Brumbies infringements.

McKellar was unsure why his side went away from what had worked in the first half and was equally perplexed about how Christie found so much space between defenders for the crucial try.

"He (Christie) did well, but we made him look very good," the coach said.

It completed a tough slog of three games across the Tasman since their gutting loss to the Queensland Reds in the Super Rugby AU final.

"In this competition you need to be really fit and healthy and we're not and a few young fellas are learning some hard lessons," McKellar said.

"New Zealand teams have gone and played at Test match intensity (against each other since COVID-19 ended the traditional Super Rugby format last year) and the Australian teams have struggled to face that.

"There's a gap there at the moment that probably wasn't there at the beginning of 2020."

Halfback Nic White completed a successful return from injury, engineering a crafty assault of the short-priced favourites in the first half.

They played close to the lines to expose the Blues' line-out and build momentum from their scrum.

That helped the Brumbies overcome an opening try to the Blues that came after a head-scratching line-out penalty that had everyone confused.

But the Brumbies then allowed the red-headed Christie to duck and dive between five Brumbies close to the line, the visitors finishing with 31 missed tackles as the Blues also dominated the breakdown.

"He looked like a ginger running from the sun looking for shade," Blues captain Tom Robinson told the broadcast post-game.

"He's such an awesome dude and it was a huge moment; that little win there got us that momentum.

"In the first half we let them in through what we call dumb-arse penalties ... and we sorted them out in the second half."