NSW Waratahs are facing a huge Super Rugby test against the unbeaten, new-look, Melbourne Rebels after their unbeaten start to the season was shattered in a 38-28 loss to the Jaguares.

The match was reminiscent of so many in the Waratahs' horrific 2017 campaign, in which they frequently conceded sizeable leads and found themselves chasing the game.

The Jaguares scored four tries in the opening quarter, including the first five-pointer within 17 seconds, to rush to a 22-0 lead at Velez Sarsfield in Buenos Aires.

Tries either side of the final hooter from replacement forward Nick Palmer and fullback Israel Folau, added a veneer of respectability to the scoreline, but had no impact on the result.

With one, last-gasp, win over the Stormers, a draw against the Sharks and now a loss, the Waratahs are already nine points adrift of Australian conference leaders the Rebels, who they host next Sunday afternoon.

Winger Emiliano Boffelli had two tries on the board inside 11 minutes, the first from the kick off, as the Jaguares carved out a 33-7 half-time lead.

"The Jaguares were very impressive at the start of the game," NSW captain Michael Hooper said. "They capitalised on opportunities and they put points on us very easily, too easily for our liking. It was hard to get back into the game."

Stung by criticism from Argentine great Hugo Porta that they were a team with no soul, the Jaguares recorded their first win in four matches, ending their worst start since joining the competition in 2016.

The Waratahs failed to convert some early opportunities of their own and couldn't shut down the dangerous off-loading game of their Argentine opponents.

After Boffelli's early brace, fullback Joaquin Tuculet cut through the NSW defence for their third try after 16 minutes and five-eighth Nicolas Sanchez touched down four minutes later to round off a fine team attack.

The Waratahs finally pierced the home defence after 35 minutes when No.8 Jed Holloway picked up from a ruck and scored under the posts. Their hopes of making further inroads into the deficit before halftime were dashed, when winger Bautista Delguy intercepted five-eighth Bernard Foley's pass and raced away for the Jaguares' fifth try.

"I felt that the Jaguares capitalised on a lot of our errors," Waratahs coach Daryl Gibson said. "There was stuff there that you'd say is largely preventable."

NSW emerged after the break intent on taking the game to the hosts but were unable to significantly slash the deficit. Winger Curtis Rona crossed in the 47th minute but centre Matias Orlando scored a sixth Jaguares try after a series of scrums close to the Waratahs line.