With wowee views of the Harbour Bridge serving as a backdrop, and white leather boardroom chairs and a bright red tea pot decorating the room, Turnbull gave a series of a year-ender interviews. As he told Fairfax Media, he had dealt with the "barnacles" of same-sex marriage, energy and citizenship that have dogged his prime ministership for much of this year. The new year will be all about tax cuts and happy days.

But despite the valedictory vibes, the 2017 political year is not over yet. Not only is there a budget update and a cabinet reshuffle to come, there's the Bennelong byelection to hurdle this Saturday.

Bennelong is not the same deal as New England, where two weeks ago, Barnaby Joyce romped home to byelection victory. Not only did Joyce face an electorate that thought it was outright ridiculous a locally born kid should be disqualified from Parliament for being a Kiwi, there were no real challengers.

Bennelong – which sits in the heartland of suburban Sydney – is an electorate that matters terribly for symbolic and very real, political reasons. It is John Howard's old seat and is officially safe Liberal (with a margin of nearly 10 per cent). But if the Liberals lose, it will mean the Coalition only has 75 seats in the lower house (including the Speaker), to Labor's 70 and the crossbench's five.

Privately, some Liberal sources are confident of a decent win on Saturday, though others are more cautious. In public, Turnbull has been eager to manage expectations. "It's a very, very tight contest," he said when campaigning in the seat this week.

Labor, while it has a spring in its step, is also refraining from predictions. "I do feel that this is going to be a tight race," Labor candidate Kristina Keneally said after a doorstop at Ryde Hospital on Wednesday. "It doesn't feel like a 10 per cent [margin] Liberal seat."

Labor frontbencher Michelle Rowland, who holds the western Sydney seat of Greenway, has been helping with the Bennelong campaign. "Anyone trying to call this now is a fool," she says, noting equal numbers of voters at a West Ryde pre-polling station this week have been taking just Labor or just Liberal how-to-vote cards.

A Newspoll published earlier this week had Liberal incumbent John Alexander and challenger Keneally on an eye-scrunching 50-50 two-party-preferred. However, it only polled 527 people and has a margin of error of 4.3 per cent. The government will take more heart from a ReachTEL poll of 819 Bennelong voters, published by Fairfax Media on Thursday, that puts Alexander clearly ahead of Keneally, 53 per cent to 47 per cent, albeit with a 3.5 per cent margin of error.

Nevertheless no effort is being spared in the last days of the campaign. Howard (who has previously been on the trail with Alexander) has recorded a robocall to Bennelong residents, Shorten was out taking selfies with Keneally before breakfast on Wednesday, while Turnbull has made repeated visits to the electorate, along with a chorus line of ministers.

Also feeding the sense of urgency is the expectation that given the distractions of Christmas shopping and school holidays starting on Friday, many people will vote before election day.

On paper, both candidates are evenly matched as star recruits who go by their initials. Before he won Bennelong for the first time in 2010, "JA" was well known to voters as a tennis champion, commentator and referee on the short-lived (but fabulous) 1990s game show, Gladiators. "KK" was premier of NSW, before transforming into a Sky News presenter.

In practice, their campaign narratives have been chalk and cheese sticks.

Those close to Alexander describe him as a personable guy who is popular in the electorate. They say he prefers the less showy side of politics: the community meetings and one-on-one greetings to the big press conferences. He has been happy just being a local member, which segues nicely into the Liberal campaign's attempts to paint Keneally as a glitzy careerist making a move for Canberra.

However, those in the Labor camp question what Alexander has achieved in the seven years he has held the seat. To be fair, he has gained some profile by speaking up on housing affordability and has organised a table tennis competition for school students. But he raised eyebrows (and thundering guffaws) on Wednesday, when he hit back at his detractors with the line: "I'm the one who installed the table tennis tables."

The ALP, meanwhile, think they have hit the jackpot with Keneally: a smart, unflappable, charismatic and highly likeable candidate. "She's good on pre-poll," Rowland notes, adding even people who don't end up voting for the Labor candidate want a photo with her.

But byelections are never just about the candidates involved. And thrown into the Bennelong mix (with its high numbers of Chinese, Korean and Indian voters) is a confusing brew of Labor senator-in-exile Sam Dastyari's Chinese connections, the government's recently announced hardline approach to foreign interference in politics, the Coalition's overall unpopularity and general voter worries about electricity prices, house prices, traffic congestion, crappy NBN experiences, schools and healthcare.

This is why no one, particularly on the government's side, is prepared to say what would be an acceptable swing against Alexander (assuming he will take some hit). As one Liberal MP says, "the barometer will actually be winning".