A Massey University study has found that the use pictures by four national newspapers during the election campaign showed substantial bias in favour of Prime Minister John Key.

The research concluded the Herald, Herald on Sunday, Dominion Post and Sunday Star-Times exhibited bias in their selection and use of images during the month-long campaign.

Associate professor Claire Robinson, who carried out the study, says Mr Key featured 138 times while Mr Goff's photo appeared 80 times.

Mr Key also dominated the column centimetres, at an almost two to one ratio.

Dr Robinson says the Labour Party has real grounds to feel it was unfairly treated in print during the last election campaign.