With scores locked at 7-7 in the 58th minute, the Brumbies hit the lead through a driving maul finished off by reserve hooker Robbie Abel before a sensational Henry Speight try extended the lead to 21-7 with 19 minutes remaining.

The winger's left-foot kick downfield was planted over the line and looked to seal the deal but moments later second-gamer Jake Gordon scored his first Super Rugby try to keep NSW in the hunt.

A tense final period ensued but it was Speight who ensured the result would be a formality as he sliced through a flimsy Waratahs defensive line with his second try in the 74th minute.

After back-to-back losses against their arch rivals last year, the Waratahs failed to capitalise on good possession in what was their third straight loss of 2017.

"I'm bitterly disappointed with that," said Waratahs captain Michael Hooper. "I thought we were a better team in the first half and sustained pressure killed us. Teams like the Brumbies who are great in that area are going to grind it out and they did."

To make matters worse, the Waratahs could be without blindside breakaway Jack Dempsey for the rest of the season after he left the field on a stretcher in the 44th minute with a suspected broken leg.

When Dempsey was being taken off the field, Fox Sports interviewed coach Daryl Gibson up in the press box and the sombre look on his face said it all, knowing that he likely won't have his best starting back-row moving forward.

"It doesn't look good for him," Gibson said. "At the moment best case, potentially is 8-12 weeks [on the sidelines], so that could be the season for him. [It is] particularly [tough] when he was coming into some pretty good form."

The wet-weather gods were against the Waratahs once again, bringing a downpour in the 48 hours before kick-off of a similar nature to the one before the Force match in round one.

Punters could have been forgiven for changing their tip mid-week, mostly because of the wet weather that generally suits the Brumbies and also the fact five-eighth Bernard Foley was ruled out on match eve.

With four dropped balls in the opening two minutes, it was clear conditions would play a major role in how both teams would go about their game.

To their credit, the Waratahs opted to kick for touch on a number of occasions rather than take easy three-pointers on offer.

It paid dividends when back-rower Scott Fardy was sent to the sin-bin after a number of infringements with the Brumbies camped on their own line.

Two minutes later, after pick-and-drives from Will Skelton and Tolu Latu, prop Tom Robertson sneaked through a hole in the Brumbies defence to put the Waratahs up 7-0 after 10 minutes.

The Brumbies were dealt a blow in the 22nd minute when hooker Josh Mann-Rea limped off the field with a suspected ankle injury but the visitors managed to negate the Waratahs for the rest of the half.

Brumbies outside-centre Tevita Kuridrani hit back with a five-pointer in the 28th minute on the back of a barnstorming run straight at Nick Phipps.

From there, both teams were happy to play field position, with NSW fullback Andrew Kellaway's kicking game a feature.

When conditions aren't in their favour and they are unable to play a free-flowing style of rugby, the Waratahs have shown in the last 12 months they struggle to modify their game. Handling errors, too, were sprinkled throughout the contest and although miserable weather could be partly blamed for this, it was hard to keep up with how many mistakes were being made.

In what was supposed to be one of the matches of the year and at a time when rugby is desperate for eyeballs, this was far from the spectacle they were after to showcase the 15-man game.

Meanwhile, there were concerns going into the match about the state of the surface and particularly after heavy downpours in recent days.

Overall, the Allianz Stadium surface held up relatively well, save for a handful of long scrums, which had the odd player lose their footing.

The Waratahs will now gear up for a match against the Rebels, in what will be billed as a battle to avoid the Australian conference wooden spoon for the time being.