The baby Brumbies answered the call when it mattered most as Ryan Lonergan drilled an 84^th minute penalty from the halfway line to claim a thrilling 27-24 win over the Melbourne Rebels at Canberra Stadium on Saturday night.

Rebels captain Matt To'omua had nailed eight penalties but the undermanned Brumbies rose to the occasion to continue their unbeaten start to the Super Rugby AU season, with Lonergan emerging as the hero of the night.

Brumbies captain Alaalatoa was red carded on the stroke of half-time for a high shot on Rebels prop Pone Fa'amausili to leave the tighthead prop walking a tightrope ahead of next week's top of the table showdown.

The Brumbies are awaiting a sanction for Alaalatoa's ill-fated tackle, while Connal McInerney will likely be sent for scans after being helped from the field early in the first half.

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To make matters worse, the Brumbies are about to meet the most serious challengers for their Super Rugby AU crown in the Queensland Reds with James Slipper and Folau Fainga'a already sidelined.

Slipper is in a knee brace following a collision at training this week, while Fainga'a has missed the past two games with a toe injury.

The potential absence of Alaalatoa, Slipper, Fainga'a and McInerney will force coach Dan McKellar to call on a band of baby Brumbies to steer the side into a clash with the Reds on home soil next week.

"That's where the game is at at the moment. Allan Alaalatoa is certainly not a dirty player," McKellar said.

"Accidents happen in the game. That's my only concern, the direction we're heading in here. It's not a stiff arm swinging to the head. You've got a big man on big man, one dropped quite late and unfortunately a shoulder hit him in the head.

"Allan plays hard and tough but he is certainly clean. I'm disappointed for Al because I know he will be disappointed, but he hasn't let anyone down tonight.

"Accidents happen, and at the moment World Rugby are pushing for any head contact to be red-carded and we've got to live with it.

"We've got confidence in one to 23, and next week we will probably have to call on a couple of others and we'll back them to step up."

Those baby Brumbies were given a taste of the action against the Rebels with 21-year-old hooker Lachlan Lonergan the man to fill the void left by McInerney after his ankle folded under the weight of Fa'amausili in the ninth minute.

The Brumbies' baby brigade was asked to answer the call at the halfway point of the contest with their team trailing 12-10 thanks to four To'omua penalty goals.

A pair of ACT young guns had already laid on the lone try of the night when Brumbies flyhalf Noah Lolesio veered towards the defensive line before putting a grubber in behind for outside centre Len Ikitau to score.

But the task ahead of McKellar's side in the second half was their toughest to date this season, the kind a 51-point drubbing the week prior won't quite prepare you for.

Law variations in Super Rugby AU meant McKellar was able to replace Alaalatoa 20 minutes after he was sent off, but he was never going to wait that long to send Tom Ross into the fray.

Jahrome Brown came from the field little more than four minutes into the second half with McKellar calling on Ross to fill the void in the front-row.

A penalty try put the hosts back in front soon after but To'omua again whittled it away with a pair of penalties to give the Rebels a sniff of their first win of the season.

The task became even tougher for the Brumbies when winger Andy Muirhead was given a yellow card with 12 minutes remaining, leaving him to watch from the sideline as To'omua nailed his seventh penalty of the night to stretch the margin to four.

A George Gregan-esque flick pass from Ryan Lonergan to put Tom Banks over for a try with eight minutes left on the clock seemed like enough to see the hosts sneak away in a thriller before To'omua added his eighth penalty to level the scores with four minutes left.

Then came Lonergan's moment in the spotlight. From 50 metres out, close to the right hand touchline, the 22-year-old handled the occasion with the class of a veteran to send the crowd into raptures.

It was the perfect tonic for a club looking to reach out to its fans leading into a grand final rematch with the Reds, while the Rebels have gone within a few kicks of beating the two competition powerhouses only to walk away with nothing.

"Suicide watch I reckon in the next couple of days. Credit to the Brumbies really, they finished the game well," Rebels coach Dave Wessels said.

"I know the wins will come. We can be tougher on each other than we probably have been. We dominated most parts of that game, we should have won.

"That's not acceptable really, from me and from the players. We've got to figure out why and we've got to change it. It's a short, sharp competition this, there's no time to really feel sorry for yourself."

AT A GLANCE

Super Rugby AU round three: ACT BRUMBIES 27 (Len Ikitau, Tom Banks, Penalty try tries; Noah Lolesio, Ryan Lonergan conversions; Lonergan, Lolesio penalties) bt MELBOURNE REBELS 24 (Matt To'omua 8 penalties) at Canberra Stadium.

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