A study has found four of the country's most-read newspapers were biased during last year's election campaign, with Prime Minister John Key receiving far more coverage than then-Labour leader Phil Goff.
Research by Massey University Associate Professor Claire Robinson, a political marketing expert in the university's College of Creative Arts, reviewed images and column widths in the New Zealand Herald, Herald on Sunday, Dominion Post and Sunday Star-Times during the campaign.
She found the papers showed "substantial bias" in their use of images - most of it in favour of Mr Key, who featured in 138 photos, while Mr Goff featured in 80.
Mr Key also dominated the column centimetres, at a ratio of almost two to one.
"Labour and Phil Goff have real grounds to feel they were unfairly treated in print during the last election campaign," Dr Robinson said.
"My research suggests there could be grounds for a complaint to the New Zealand Press Council that the newspapers breached the principle of fairness and balance in their campaign coverage."
Dr Robinson said both leaders received much more positive and neutral coverage than negative coverage from all four papers, but the Herald and Herald on Sunday were generally more positive in their treatment of Mr Key, while the Dominion Post and Sunday Star-Times were kinder to Mr Goff.
During the teapot tape saga, where a conversation between Mr Key and ACT leader John Banks was recorded following a photo opportunity, just days out from the election, Dr Robinson found "a dramatic shift" in image selection by the newspapers - which began to run highly unflattering shots of John Key.
In the final days of the campaign, none of the papers published a negative image of Mr Goff, but before the teapot tape scandal, more than one in three images of him were negative.
She said newspaper editors need to think again about the impact of images, not just words, on their audience.
"It is time they paid serious attention to the ethics of which images they publish, when and how."