The ABC's election analyst, Antony Green, says it is likely there will be a swing away from the National party at this weekend's Clarence by-election.

Saturday's by-election is to replace former MP Steve Cansdell, who resigned in September, admitting he had falsified a statutory declaration to avoid a speeding fine.

Mr Green says the Nationals are expected to win the seat but there will probably be a significant swing to Labor.

"The main reason there'll be a swing is that the Labour Party put up little more than a token effort at the general election," he said.

"They are putting considerable effort into the by-election and the past has shown when Labor bothers and puts a bit of effort in they give the Nationals a bit of a run for their money.

"Which is why the result will be better for Labour than the general election and will look like a substantial swing, when really it's just a reversion to a more normal looking voting pattern."

Mr Green's prediction is that Labor and the Greens will do better in the by-election than they did in the March general election.

"I think it's highly expected the National party will win the seat but the question is what sort of swing occurs at the election," he said.

"Labor had a terrible result at the state election, so you'd expect some substantial bounce back for the Labor Party at the by-election.

"There'll be a substantial swing to Labor.

"I don't think it's going to endanger the seat.

"If it does endanger the seat it'll be a really nasty shock for the National party."

Follow Anthony Green's coverage of the Clarence by-election here.