Former ACTU boss Ged Kearney emerged the winner on Saturday with 42 per cent of the primary vote, 52 per cent on a two-party basis, trumping the Greens candidate Alex Bhathal who has now lost the seat six times.

Ms Kearney, who does not live in the inner-metropolitan electorate, is yet to attend the Labor by-election party to make her victory speech, but Greens leader Richard Di Natale has conceded defeat.

The mood was electric at the Labor camp in Thornbury, with supporters wearing red chanting 'Ged, Ged' and 'solidarity forever' as results poured in, though the count remained neck-in-neck for some time.

Federal Labor MP Maria Vamvakinou was confident the party would retain the long-held seat, left vacant by Labor's David Feeney after he resigned amidst the citizenship saga.

She said that was largely due to Ms Kearney's work history, as a nurse and unionist, and warm character.

"I think all that made a big difference in terms of engaging people locally and perhaps getting them to refocus on Labor and a Labor agenda," the Member for Calwell told AAP.

"I'm feeling confident that Ged has done everything possible to maintain the seat today."

Mr Feeney was also at the party, but did not wish to comment.

Opposition Leader Bill Shorten earlier said Labor had faced a tough fight.

But he said Ms Kearney helped put Labor in a very competitive position.

"She represents real change for the seat and real change for the lives of many of the people in this seat," Mr Shorten told reporters earlier on Saturday.

Ms Kearney said Labor had run an excellent campaign.

The Liberal Party did not run a candidate in Saturday's by-election.

Two-party preferred, there has been a swing to Ms Kearney of just over one percent, while Ms Bhathal experienced a swing against her of the same margin.

Ms Bhathal, who has denied allegations against her of bullying and intimidation, claimed 41 per cent of the primary vote, with about 48 per cent on a two-party preferred.

Australian Associated Press