The ACT Brumbies have secured a Super Rugby Pacific finals spot and placed the NSW Waratahs' season on life support with a clinical 21-14 derby win in Sydney.

The Brumbies punished the Waratahs for their lack of execution, with 37-year-old lock Cadeyrn Neville proving an unlikely hero in Friday night's tense victory at Allianz Stadium.

While Australia's benchmark Brumbies also kept alive their hopes of a crucial top-four finish, another frustrating home defeat for the Waratahs left Dan McKellar's erratic outfit needing a miracle to make the playoffs.

The Tahs must beat the Western Force in Perth next Saturday and pray the sixth-placed Queensland Reds lose their remaining two regular-season games - against Moana Pasifika in Auckland and Fijian Drua in Brisbane - to scrape into the playoffs.

Finishing in the top four shapes as critical under the new finals format, which pits first versus sixth, second against fifth and third versus fourth in the opening week of the playoffs.

The highest-ranked losers in the first three finals - which are all likely to be staged in New Zealand - will advance to the semi-finals.

If the Brumbies beat Moana Pasifika in Canberra and the Crusaders lose to the table-topping Hurricanes in Christchurch, Stephen Larkham's team will snatch fourth.

The Brumbies could conceivably finish third and snare a home final if the second-placed Blues also lose to the second-placed Chiefs next weekend.

For now, though, they can celebrate sealing a finals berth and regaining bragging rights after avoiding losing twice to the Waratahs in a single season for the first time since 2015.

NSW will be cursing themselves after dominating first-half possession and territory but blowing numerous try-scoring chances to somehow be trailing 14-0 at the break.

Neville was the Waratahs' nemesis.

First he defied age and physics to pull down Jack Bowen just as the Tahs flyhalf seemed certain to score.

Then he crossed for the Brumbies' second try, after scheming scrumhalf Ryan Lonergan put hooker Billy Pollard over for their first against the run of play in the 22nd minute.

Lonergan also looked set to score before his opposite number Teddy Wilson pulled off a desperate try-saver, only for Waratahs flanker Charlie Gamble to be yellow-carded for offside intervention in the ensuing play.

The Brumbies wasted no time doubling their lead through Neville's try a minute later.

The visitors looked home and hosed when prop Allan Alalaatoa celebrated his 150th game with a rare try to take the visitors out to a 21-0 lead.

But late tries after the hour mark to Waratahs prop Isaac Kailea and Max Jorgensen set up a grandstand finish.

ACT Brumbies 21 (Tries: Pollard, Neville, Alaalatoa; Cons: R. Lonergan 3) def NSW Waratahs 14 (Tries: Kailea, Jorgensen; Cons: Harvey 2)

Teams:

Waratahs: 15 Max Jorgensen, 14 Harry Potter, 13 Triston Reilly, 12 Lawson Creighton, 11 Sid Harvey, 10 Jack Bowen, 9 Teddy Wilson, 8 Angus Scott-Young, 7 Charlie Gamble, 6 Clem Halaholo, 5 Miles Amatosero, 4 Matt Philip (captain), 3 Dan Botha, 2 Ioane Moananu, 1 Jack Barrett.
Replacements: 16 Oniti Finau, 17 Isaac Kailea, 18 Apolosi Ranawai, 19 Ben Grant, 20 Jamie Adamson, 21 Michael McDonald, 22 Jack Debreczeni, 23 Joey Walton

Brumbies: 15 Tom Wright, 14 Ollie Sapsford, 13 Kadin Pritchard, 12 David Feliuai, 11 Corey Toole, 10 Declan Meredith, 9 Ryan Lonergan (captain), 8 Toby Macpherson, 7 Rory Scott, 6 Rob Valetini, 5 Lachlan Shaw, 4 Cadeyrn Neville, 3 Allan Alaalatoa, 2 Billy Pollard, 1 James Slipper.
Replacements: 16 Lachlan Lonergan, 17 Blake Schoupp, 18 Rhys van Nek, 19 Nick Frost, 20 Luke Reimer, 21 Klayton Thorn, 22 Tane Edmed, 23 Andy Muirhead.

Referee: Ben O’Keeffe
Assistant referees: Todd Petrie, Warwick Lahmert
TMO: Richard Kelly