Voter unhappiness in the formerly safe NSW Liberal seat of Willoughby goes back at least three years, and the swing in last Saturday's by-election should be "no surprise", the independent challenger says.

Larissa Penn said when she ran against former premier Gladys Berejiklian in 2019 there was already "growing discontent across a range of issues" in the northern Sydney electorate.

"I was getting contacted every day by the community (urging me) to run," she told AAP.

"I'm involved in lots of different community groups, environment groups, P&Cs, advisory groups to council," she said, adding her backers were also heavily involved in the community.

The most recent count puts Liberal candidate Tim James ahead at 51.8 per cent, with Ms Penn at 48.2 per cent in two candidate preferred terms, with a swing against the Liberals of 19 per cent.

Ms Penn said Willoughby voters have a range of concerns, and want politicians to make "evidence-based decisions" that put public interest first.

She said locals had "great difficulty" accessing information like contamination reports and businesses cases, which informed how projects like the second Harbour tunnel and Beaches Link would impact their community.

"We've had to lobby very hard just to get the basic information released to the community and we're still having that issue," she said.

Respect for nurses and teachers had also become a key issue as the electorate was a large catchment for schools and hospitals, including Royal North Shore.

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