Prime Minister Jacinda Ardern says New Zealand has directly raised concerns with China over a tweet criticising Australia with a falsified image.

Australia and China are in the middle of a spat over a tweet posted by a senior Chinese official concerning the recent report on war crimes alleged committed by Australian troops in Afghanistan.

The tweet showed a falsified image of an Australian soldier holding a knife to the throat of an Afghan child.

Australian Prime Minister Scott Morrison has demanded an apology and called it a “disgusting slur”. China’s Foreign Ministry spokesman, Hua Chunying, instead responded to say Australia should apologise to the Afghan people.

“What Australia should do is to reflect deeply, bring the perpetrators to justice, make a formal apology to the Afghan people, and solemnly promise to the international community that they will never commit such terrible crimes again,” Hua said.

New Zealand is in a tricky position, with its closest ally, Australia, locked in a war of words with its largest trading partner, China.

Ardern said New Zealand had registered its concern over the “unfactual” image directly with China.

“I’ve seen some of the latest discussion over what was a Twitter image. New Zealand has registered directly with Chinese authorities our concern over the use of that image. It was an unfactual post, and of course that would concern us,” Ardern said.

“In this case an image has been used that is not factually correct [and] that is not a genuine image. So we have raised that directly with Chinese authorities.”

She said New Zealand did this independently in its own interest and she had not spoken to Morrison about the spat.

“In keeping with our principled position we will raise concerns, and we will do it directly.”

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Opposition leader Judith Collins said she had not yet fully considered how New Zealand should react to the spat.

“It’s not something I’ve turned my mind to ... The Government always needs to take an independent foreign policy,” Collins said.

“We’ve had a proud history for the last 30-odd years of trying to steer an independent way through, but understanding that – and I’ve always said that to people – one of the best things for New Zealand is to have multiple trading options, as well as our very good security options,” Collins said.

The National Party’s foreign affairs spokesman, Gerry Brownlee, said it was an “awkward situation that is not at all comforting for anyone” and that “everyone should take a breath and move on a bit”.

“It was inappropriate for China to doctor the photograph of the Australian military person,” Brownlee said.

He said the spat was concerning for New Zealand, which could get caught up in it.

Brownlee said New Zealand’s ties were stronger with Australia than China.

An investigation by Australian authorities into war crimes allegations by its soldiers found that there was substantial evidence behind allegations of 39 unlawful killings of Afghan civilians. At least 19 Australian former and current soldiers are now being investigated by police.