A predicted Labor victory would be its first in Bega, held by the Liberals since its creation in 1988 and occupied by retiring member Andrew Constance since 2003.

Opposition Leader Chris Minns previously said the seat would be "almost impossible" for his party to win, but early tallies in the south coast electorate on Saturday night gave a swing of more than 14 per cent to the ALP's Michael Holland over the Liberal Party's Fiona Kotvojs.

Dr Holland says the "incredible" swing to Labor in Bega is a reflection of the community's response to the current government.

"It's also been a plebiscite ... to wanting a local member who will represent them with integrity and honesty," Dr Holland said on ABC News.

Premier Dominic Perrottet said on Sunday the government will reflect and learn from "some disappointing results across the board".

There's certainly lessons to be taken from the "particularly disappointing" result in Bega, but by-elections are notoriously hard for governments, Mr Perrottet said.

With votes from just under a third of the electorate counted as of 10.45pm on Saturday, Dr Holland had received 57.1 per cent of two-candidate preferred votes, compared to Ms Kotvojs' 42.9 per cent, according to the NSW Electoral Commission.

The Liberals went into the by-election with a margin of 6.9 per cent.

A likely loss in Bega will push the government further into minority and force it to rely more heavily on the votes of independent or minor party MPs.

Mr Minns says he and Dr Holland were "straight back to work" on Sunday morning, discussing plans for the Eurobodalla Hospital development, which the premier said his government was already building.

Labor candidate Jason Yat-Sen Li looks set to win Strathfield, leading Liberal candidate Bridget Sakr 54.4 per cent to 45.6 per cent with about 37 per cent of the electorate's votes counted.

Mr Perrottet insisted Labor's vote had gone backwards in the electorate and vowed Ms Sakr would win the election next year.

The Nationals are on track to hold former deputy premier John Barilaro's seat of Monaro despite a swing of more than six per cent.

With about 44 per cent of the vote counted, Nichole Overall led Labor's Bryce Wilson 55 per cent to 45 per cent.

Two-candidate preferred results for Willoughby, which Labor did not contest, are not yet available.

Liberal Tim James has so far secured about 43.4 per cent of first preference votes, trailed by Independent Larissa Penn on 32.1 per cent, with just over 40 per cent of the blue-ribbon electorate's votes counted.

The by-elections were viewed as a test for the premier and his government's handling of the state's Omicron wave.

The stakes were 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.

Mr Minns said voters had sent the premier a message on Saturday that the government needs to change direction.

Close to 40 per cent of constituents voted early or via post, so results are not expected to be finalised for some time.

The NSW Electoral Commission says no votes will be counted on Sunday.

Counting will resume on Monday, and postal votes will be counted from next weekend, with final results not expected until later this month.

Australian Associated Press