A diabolical start cost the ACT Brumbies any chance of a spot in the Super Rugby final as they were thrashed 39-7 by the Jaguares in Buenos Aires on Saturday morning (AEST).

The Jaguares were playing in their first semi-final but showed no sign of nerves in front of a fever-pitch crowd of 31,000 where the hosts were almost flawless in the first half.

It was always going to be tough for the Brumbies against a team boasting 14 Test players in their starting side, and a further 185 international caps on the bench.

But the visitors simply didn't turn up and looked lacklustre from the opening whistle as the 30-hour trip from Canberra to Argentina appeared to have taken a toll.

"I'm really disappointed in that one," said Brumbies skipper Christian Lealiifano.

"They were very good tonight and put us under a lot of pressure. We just couldn't get back on top. We got in late in that first half but the Jaguares were very good."

The Brumbies were riding a seven-game winning streak heading into the contest, but that scintillating form which took them to the finals was nowhere to be seen.

Brumbies hooker Folau Faingaa had his worst game of the season and lost an inconceivable five lineouts in the opening 25 minutes.

The Jaguares scored the opener after a mix up between Rob Valetini and Toni Pulu saw a chip kick deflect through the last line of defence and former Brumbies halfback Tomas Cubelli swooped.

It proved a sign of things to come.

Joaquin Diaz Bonilla booted back-to-back penalties, both won after lost Brumbies' lineouts as the Jaguares raced to a 13-0 lead.

The Brumbies' polish of the past two months had well and truly worn off, with the visitors making basic errors as the clinical Jaguares outfit made most of their chances.

The next mistake came from No.8 Lachlan McCaffrey, when he was caught isolated on the blind side and the Jaguares punched the corner from the ensuing penalty.

A few phases later and lock Tomas Lavanini barged over to score. Just like that, it was 20-0 after 20 minutes.

The Brumbies scored a crucial try through Faingaa on the stroke of half-time, but it proved a false dawn as the mistakes continued in the second half.

Lealiifano, playing his 150th and final game of Super Rugby, tried to grubber through on 50 minutes, but it was defused by Matias Moroni who sprinted 50 metres before finding Matias Orlando to score.

Orlando burst through some tired defenders to cross for his second on 62 minutes before fullback Emiliano Boffelli scored a turnover try in the dying moments.

Bonilla finished with a perfect six from six off the tee.

The Jaguares have now won 11 of their past 12 games and will face the Crusaders or Hurricanes in the final.

It marks their first decider in just their fourth season of Super Rugby.