Senior NSW Labor sources have said they will lose the state's most marginal seat, East Hills, after a smear campaign alleged its star candidate was a supporter of paedophiles.

Corflutes for the respected Cameron Murphy have been plastered with stickers maliciously alleging he is a "paedophile lover".

The Labor candidate for East Hills Cameron Murphy. Credit:Lee Besford

Mr Murphy was awarded the Order of Australia in 2014 for his contribution to human rights, and is the son of former Labor federal attorney general and former High Court judge Lionel Murphy.

"It has been the dirtiest campaign on a local level that NSW has ever seen," said a NSW Labor source.