The reigning champions have never been beaten by a domestic rival and kept the impressive record alive at Canberra's Viking Park in their Sunday clash.
NSW got off to a flying start and led 26-7 at halftime, with Wallaroos lock Leaney chiming in with two tries in her 25th match for her side.
Her second was scored in extra time in the half, with the visitors taking advantage of the Brumbies losing prop Martha Fua to a yellow card for collapsing a maul.
"It was a tough one out there. The first half was really exciting for us and we played in the right areas and had some really exciting attacking rugby," Leaney told Stan Sport.
"In that second half a couple of discipline errors got us on the back foot and the Brumbies really came out firing and it was a battle out there."
Skipper Emily Chancellor also touched down after running on to a Piper Duck pass, while the Waratahs used their set piece strength to put hooker Brittany Merlo over the line.
Former Queensland winger Ivania Wong got on the scoreboard for the Brumbies in the 24th minute after they worked the ball wide.
The second half was a more even affair with hard-running Brumbies blindside flanker Lily Bone closing the margin with a try four minutes in.
They added to that tally in the 75th minute through Spanish scrumhalf Anne Fernandez de Corres.
While the lead proved too much to peg back, the Brumbies can take pride from keeping NSW from adding to their try-scoring haul, with a late penalty from Desiree Miller their only second half points.
In Saturday's clash in Fiji, the Drua rallied from 14 points down in the second half to over-run the Western Force 29-24.
Meeting at Churchill Park in Lautoka, the Force looked like banking a comfortable victory when Wallaroos scrum-half Sammy Wood converted her own 51st minute try for a 29-10 lead.
But the hosts scored three tries in 12 minutes with No.10 Varanisese Qoro crossing for a double.
The Force almost managed to wrest back the lead but were denied by the Drua, shutting down a five metre line-out with less than two minutes on the clock.
After a first round bye, the Force return to Perth and will meet the Reds in round three.
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