Four months after former premier Gladys Berejiklian's sudden resignation prompted three other MPs to follow suit, polling day finally arrived on Saturday in Bega, Monaro, Strathfield and Willoughby.

While about 40 per cent of constituents in the electorates had already picked their candidate - voting early or via post - people turned out to booths saturated with signage throughout the morning.

Premier Dominic Perrottet made campaign pit stops in both Sydney seats, Strathfield and Willoughby, while Opposition Leader Chris Minns started his morning in Bega.

Prime Minister Scott Morrison gave a shout out to "dear friend" Bridget Sakr vying to win for the Liberals in Strathfield, as federal Labor leader Anthony Albanese lent a hand in pitching for votes on the ground in the same electorate.

Former deputy premier John Barilaro was also out spruiking for the Nationals candidate he backed to replace him in Monaro.

However a notable absence on polling day was Ms Berejiklian.

Liberal candidate Tim James is tipped to be her successor as member for Willoughby, a safe blue-ribbon seat on Sydney's north shore.

It's another story in Bega, where Labor is hoping it can net a "historic win".

The seat on the state's south coast was held by Liberal and former transport minister Andrew Constance on 6.9 per cent margin.

Labor has never won it but is hopeful local gynaecologist and obstetrician Michael Holland can break the drought.

Mr Minns painted the battle in the electorate as a referendum on the new premier.

"If a conservative candidate is re-elected in this seat, and in the seat of Strathfield, it'll give a green light to all of the policies of Dominic Perrottet," he said.

"Voters recognise that you can't change the government in a by-election but you can send a message.

"I think many people want to put a handbrake on the government."

However he was quick to clarify the by-election results were not necessarily a reflection on his leadership ahead of the general election in March 2023.

Dr Holland - who said his success would be a "historic win" - is running against farmer and former teacher Fiona Kotvojs for the Liberals.

But the Liberals are also threatening a Labor seat, won on a margin of five per cent at the last election.

Liberal candidate Ms Sakr - whose 11-year-old daughter died along with three other children when they were hit by a drunk and drugged driver at Oatlands in 2020 - is hopeful she'll be next member for Strathfield.

Upset over the departure of former Labor leader Jodi McKay, who many feel was treated unfairly by the party, could help carry her over the line.

Former United Nations lawyer Jason Yat-sen Li hopes to retain the seat for the Labor, with former prime minister Kevin Rudd previously lending his weight to his campaign.

Elsewhere, Nationals candidate Nichole Overall and Labor's pick Bryce Wilson are facing off in Monaro - held by the Nationals on a margin of 11.6 per cent.

The contests are the first electoral test for Mr Perrottet and Mr Minns.

Both were recently elevated to their position and both are painting their parties as the underdogs.

For Mr Perrottet, there is the usual swing against the government in by-elections, the loss of popular and senior MPs, as well as scrutiny of his handling of the surging Omicron wave.

The government is already in minority, and a loss of any further seats would force it to rely more heavily on the votes of independent or minor party MPs.

The stakes are also high for Mr Minns, who has pitched himself as the man to make Labor a real election chance after more than a decade in opposition.

Although some results will filter through on Saturday night, the NSW electoral commissioner says it could be a month before final outcomes are known, thanks to the high postal vote.

Australian Associated Press