Nationals candidate Adam Marshall has taken a landslide victory in the by-election for the NSW seat of the Northern Tablelands, which was previously held by independent Richard Torbay.

Figures on Saturday night showed Mr Marshall had more than 60 per cent of the vote.

Independent Jim Maher won 14 per cent of the vote, while just under 10 per cent voted for Labor's candidate, Herman Beyersdorf.

The result gives the O'Farrell Government a 70th seat in the Legislative Assembly.

The by-election came after Mr Torbay's spectacular fall from grace earlier this year after 14 years in politics.

He was referred to the Independent Commission Against Corruption and was disendorsed as the Nationals candidate in the federal seat of New England, where he had hoped to topple sitting independent MP Tony Windsor.

ABC election analyst Anthony Green says the strong result for the Nationals will spark speculation about Barnaby Joyce's chances of beating Mr Windsor during the federal election in September.

Mr Joyce gave up his Queensland Senate seat to run for the Nationals in New England.

"Clearly this is a very strong National vote and the party will be hoping to repeat the vote with Barnaby Joyce in New England at the Federal election in September," Green wrote on his election blog.

"However, New England also includes Windsor's home base in Tamworth, and Windsor is clearly a more formidable opponent than either Jim Maher or Herman Beyersdorf at the by-election.

"Even Windsor's biggest haters would have to concede that point. Tony Windsor was not on the ballot paper, so a direct transfer of these results to a federal election is not valid.

"However, it is a terrible result for the state Labor Party, not even making double figures.

"In part, this is because of Jim Maher being on the ballot paper, but there must be some teeth gnashing over this result by both the state and federal Labor parties."

Mr Marshall first entered politics at the age of 19 when he was elected to the Gunnedah Council in 2004.

At 28, Mr Marshall will be the youngest member of the state parliament.

Earlier on Saturday, former LNP heavyweight Barry O'Sullivan was selected to replace Mr Joyce in the Senate.