The mood was glum and the faces of the Labor faithful mournful at the Thornbury theatre just over three weeks ago.

The party had just been routed in the byelection for the Victorian seat of Northcote on the back of an 11 per cent swing to the Greens.

This was despite a major pitch from Australia's most progressive premier, Daniel Andrews, and a well-resourced and targeted campaign.

Among the mourners was David Feeney, the federal member for the overlapping seat of Batman.

No doubt he was thinking about what the Greens thumping would mean for his future, and it now looks like he won't have to wait long.

Mr Feeney has been caught up in the Parliament's citizenship saga, and has asked the High Court to consider his matter, after being unable to find documents showing he renounced UK citizenship.

Labor's nightmare in the north is in serious danger of being repeated, with Batman looming as the only seat in the country the party would be at major risk of losing if a by-election is called.

The prognosis from Labor sources is bleak.

"We're cooked" said one.

"No chance, we're f****ed" said another.

And the Greens' performance in Northcote suggested the minor party was well-placed to win its second lower house seat.

Labor will take comfort that at the last federal poll Mr Feeney won despite having a catastrophic campaign, after it was revealed he did not live in the electorate and had an undeclared negatively-geared property in Northcote, which was rented out to Greens voters.

He managed to hold on 51-49 per cent in the two-party-preferred vote, despite the Greens pipping him in the primary vote.

The Liberals won nearly 20 per cent of the primary but are unlikely to run at a byelection if Northcote is a guide, making victory for Labor tougher.

Labor analysis showed that despite the savage loss in Northcote, the party's primary vote in the area went up seven per cent compared with Mr Feeney's 2016 performance.

A former ALP stronghold

Like many inner city seats, Batman was once a Labor stronghold.

In 2010, Martin Ferguson won with 57 per cent of the two-party-preferred vote, and three years ago Mr Feeney won his first election with a margin of 10 per cent.

But a changing demographic and soaring house prices has seen a dramatic shift in the electorate.

Greens candidate Lidia Thorpe easily won Northcote in a state by-election last month. ( ABC News: Chris Sonesson )

It has become strongly progressive and many residents do not rely on government, leaving voters in the area focused on bigger picture issues such as the environment and refugees.

The treatment of asylum seekers — especially with the ongoing situation on Manus Island — looms as the issue that could break Labor's back in Batman.

The Greens managed to steal Northcote at state level but in a federal contest their brand is even stronger.

It is easier for the Greens to make a stronger pitch to swinging left-wing voters when Bill Shorten is the proposition compared to Mr Andrews.

The Greens have already preselected Alex Bhathal, who is running for the sixth time.

News of Mr Feeney's citizenship calamity was greeted with anger and little surprise from within the party, with people pointing out that in 2007 he told the vetting process that he had renounced his citizenship, but there was nothing to prove it.

As one senior Labor source said "It's the perfect storm".

And for Greens voters, the prospect of increasing crossbench influence in an already precarious parliament is likely to sway votes.

A new candidate?

The Labor brand in progressive Northcote has not done well under Mr Feeney, with the Greens trying to paint him as a factional player with little connection to voters in the area.

There has been a dramatic shift in the demographic of Batman, once an ALP stronghold. ( ABC News: Chris Sonesson )

This angle was even used against state candidate Clare Burns last month, and feedback from the area was that his factional appointment to the seat after Martin Ferguson's retirement did not help the ALP cause at last year's state poll or the by-election

There have been some early murmurings that perhaps the party would look for another candidate in the event of a by-election, and given the tight time frame, any such move would likely be at the discretion of the national executive and Mr Shorten.

"A good incumbent is worth a couple of per cent, Feeney is probably a negative,'' one senior figure said.

Of course all of this is hypothetical. The MP has not yet been referred to the High Court and a by-election has not been called.

Labor lost Northcote after starting with a six per cent lead. In Batman, the lead's just one per cent, so it if a by-election is called, Labor's northern nightmare might just be re-lived.