The Greens are on track to seize another Lower House seat in the Victorian Parliament, according to internal Labor party polling ahead of an inner Melbourne by-election.

The polling shows primary support for the ALP in the seat of Northcote below 30 per cent.

Key points: Late Labor MP Fiona Richardson held Northcote by 6 per cent until changes in demographics saw Greens surge

Late Labor MP Fiona Richardson held Northcote by 6 per cent until changes in demographics saw Greens surge Loss of Northcote would mean ALP would hold slim one-seat majority in Victorian Parliament

Loss of Northcote would mean ALP would hold slim one-seat majority in Victorian Parliament Greens have picked Indigenous candidate Lidia Thorpe to take on ALP's Clare Burns

The November 18 by-election was triggered by the death of minister Fiona Richardson and pits Labor against a strong Greens threat.

Ms Richardson held the seat by 6 per cent, but changes in demographics and house prices have seen the Greens' popularity surge across once-safe Labor seats in the inner city.

The ABC understands polling in the seat shows the Greens on track to win, with 40 per cent of the primary vote.

Labor has just 28 per cent of the vote, and 17 per cent of voters are undecided, according to the polling.

It was conducted by the ALP to test the level of primary support and was presented to the ALP Campaign Committee recently.

The data is used to work out where to best target campaign resources and strategies.

The campaign for the by-election is long, designed to give Labor maximum time to talk to, and target, as many voters as possible.

The party is spending hundreds of thousands of dollars on the campaign and is pulling out all stops to try to hold the seat, which has been in ALP hands since it was established in 1927.

The ALP's Clare Burns is a Trades Hall organiser. ( Supplied )

A loss would see Labor hold a wafer-thin majority in the Lower House, although it can still rely on independent Don Nardella, a former Labor MP, for support on most bills.

At the 2014 Ms Richardson won 41 per cent of the primary vote in Northcote. ( Supplied: Victorian Electoral Commission )

Labor's candidate is Trades Hall organiser Clare Burns and the Greens have picked Indigenous woman Lidia Thorpe, who is managing director of a business that provides sustainable housing and renewable energy to remote communities.

The Liberal Party has decided not to field a candidate. Its preferences have helped the Labor Party in the past.

The Australian Conservatives is also not fielding a candidate, despite some speculation the party might run to test the levels of support.

The party's preferences could have been critical to help Labor win.

Labor's poor polling in Northcote comes after Labor MP Jane Garrett announced she was quitting Brunswick to move to the Upper House. Ms Garrett cited a battle with breast cancer as a motivation to move to the Upper House.

ACTU president Ged Kearney has been drafted in to run for Brunswick, suggesting her backers believe Labor is still in a strong position.

The death of Fiona Richardson meant Northcote went to a by-election. ( AAP: Julian Smith )

Brunswick, along with neighbouring Richmond, presents the same Greens threat for the ALP.

The Greens won their first two Lower House seats at the last election, with Ellen Sandell winning Melbourne off Labor and Sam Hibbins winning Prahran off the Liberals.

The party also has five Upper House spots.

In the 88-member house, Labor currently has 45 seats (but one of those is held by the Speaker), the Liberals hold 30 and the Nationals hold seven.

Three independents share the crossbench with the two Greens.

A Labor loss in Northcote will heighten fears of more inner city losses at next year's state election, but ALP sources are more positive about the party's prospects in Richmond and Brunswick.

Greens wins in those seats would increase the need for the Government to win seats off Matthew Guy's Coalition to retain office in 2018.

They could also result in a hung Victorian Parliament after the election on November 24 next year.