Katelin McInerney has pledged to work for the entire electorate of Kiama after her landslide victory in Saturday's by-election.

The Labor candidate was declared the winner less than two hours after the polls closed after enjoying a huge swing in her favour after narrowly missing out on victory last time around to former independent MP Gareth Ward.

His resignation after being jailed over sexual assault offences was always going to make it difficult for the Liberals to win this election after he had represented the Party for more than a decade.

Liberal candidate Serena Copley trailed McInerney 60-40% after more than 44,000 votes were counted on Saturday night, meaning Labor regains the seat for the first time since 2011 and the former journalist and union campaigner has become the first female to represent the electorate.

The key to McInerney's stunning victory was a large proportion of the 20,000 people who voted for Ward last time around switching their allegiances to Labor.

It was the first time a NSW government has won a by-election seat since 1996, which is bad news for Opposition Leader Mark Speakman's hopes of retaining the role in the lead-up to the state election in early 2027.

"Many people voted Labor for the first time in a long time. I just want to say I thank them deeply for putting their faith and their trust and their confidence in me and in the NSW Minns Labor Government. Whoever you've supported in the past, whoever you chose to support today, it doesn't matter," McInerney said.

"I will be a strong advocate and a strong voice in the NSW Parliament on your behalf, representing this community every day.

"The work starts today. I promised that I would be a strong voice inside government, and it starts right here, and that's what I'll be.

"And we will continue to deliver on the things that we were out there talking about (on the campaign trail)."

McInerney said she comes from "a family of Labor tragics".

"The reason that you all received a sandwich today was because that's a tradition that dates back to my grandfather, who was a delegate for the TWU," she said at Albion Park Bowling Club on Saturday night to the delirious Labor faithful.

"He and our family would deliver those sandwiches anywhere in Sydney they needed to go.

"If it was Randwick or Ryde, they delivered those sandwiches because you know what's more important than a good feed? The look on the opposition's face when you show up with a free sandwich.

"That's what being part of Labor is all about. We are a team. We back each other and we are all about that solidarity."

The Kiama electorate has been neglected somewhat in recent years with the Government freezing Ward out after he refused to resign despite facing serious criminal charges.

McInerney said the future now looked bright for the region.

"We are leaving no one behind in the electorate of Kiama anymore," she added.

"No one telling me that they haven't had the infrastructure investment or the investment in their essential services that they needed.

"No one telling me that their public school needs work. We are going to be working every day. I'm going to be working every day to make sure that our electorate gets the infrastructure, the investment, the essential services it deserves going forward."

NSW Premier Chris Minns on Sunday said Labor had run a "shrewd and energetic election campaign".

"The circumstances were very grave and serious that led to the by-election but to be honest with all of you, it was very advantageous to the Labor Party that a by-election fell due," he said.

"The incumbent member of parliament is sitting in jail and I think it's important that I note that those circumstances will not be repeated in another by-election or certainly in the next general election.

"And we need to be aware of that and approach this result in a humble way. I think that the clear message from the electorate was that they expect us to work even harder and the point I'm trying to make is if there was an election held in different parts of NSW, I can think of off the top of my head not only would we not have won I think we would have gotten beaten and that's what I'm saying about this election campaign.

"It's quite patchy across NSW. My sense is the voters of NSW expect us to make major inroads and real progress at the challenges that they're facing in their community and if we don't make real progress then we will not win the 2027 election campaign."

With McInerney attracting 36% of the vote, Copley was a long way back on 25.2% with Community Independent Kate Dezarnaulds further adrift in third spot with 10.7.

With a total of 13 candidates on the ballot paper, The Greens' Tonia Gray has only received 7.8% of the diluted vote ahead of Don Fuggle (Legalise Cannabis Party) and Felix Nelson (Shooters, Farmers, Fishers) who had 4.9% each.

None of the other seven candidates cracked the 2% barrier.