De Vanna scored a first-half double and the second strike was a contender for W-League goal of the year, but it was her celebration which raised even more eyebrows.

The ageless 33-year-old showed blistering pace down the right flank and moved into the box before cutting back inside and curling a pinpoint strike into the top left hand corner on 42 minutes.

De Vanna then sprinted to the technical area and celebrated with her manager Ante Juric, before turning her attention to Canberra coach Heather Garriock.

De Vanna walked to within three metres of her former Matildas teammate and then planted her feed, standing arms folded and chest out while staring down Garriock as the pair exchanged words in a heated moment.

"We're best mates, she played for us last season and now she's playing for my old club and that was just her trying to be smart with me," Garriock said.

De Vanna was a marque signing for Canberra last season under former coach Rae Dower and her player-of-the-match performance earned a standing ovation when she left the pitch following a 69-minute shift.

United trailed 2-0 at the half and things went from bad to worse after the break when American import Toni Pressley scored what will go down as one of the worst own goals in the history of the W-League.

Two minutes into the second stanza De Vanna hoofed a hit-and-hope ball into an empty box and with no Sydney players in sight, Pressley inexplicably headed the ball into goal as United keeper Haley Kopmeyer watched on helplessly.

In high-tempo opening 15 minutes neither team gave an inch and on 27 minutes Nicky Flannery fell victim to the physical contest as she left the field in tears with a leg injury.

The Sky Blues broke the deadlock on the half hour mark when Princess Ibini-Isei sent a bullet cross to an unmarked De Vanna lurking at the back post and she made no mistake from close range.

After the wonder strike and own goal, Garriock boldly sent players forward trying to mount an unlikely comeback victory, and despite chances falling to Michelle Heyman and Grace Maher, the goals never came.

It was only Kopmeyer who stopped Sydney racking up a cricket score as the American made yet another series of brilliant saves.

It marked three losses on the bounce for United and they could head into their three-week Christmas break languishing in seventh place after pushing for top spot just a month ago.

"I assembled this team so this is on me, I've never been in this position before, I'm used to winning so I need to go away and assess where we're at as a team," Garriock said.

"I need to have a think if I have the right personnel to take this side forward but either way we've got the same squad for the rest of the season so I just need come up with some answers over the break."

SYDNEY FC 3 (De Vanna 30' 42' Pressley OG 48') bt CANBERRA UNITED 0.