This photo shows the Al Noor mosque in Christchurch, New Zealand. File photo

New Zealand police said on Wednesday they have stepped up patrols at two mosques ahead of the first anniversary of a shooting that killed 51 worshippers, after a distressing image linked to one of the mosques appeared on social media.

A lone gunman used a semi-automatic gun to kill worshippers gathered for Friday prayers at two mosques in the city of Christchurch on March 15 last year in New Zealand's worst peace-time shooting.

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The attack prompted the government to tighten gun laws and raised questions about the country's image as a peaceful, law-abiding society.

Mourners lay flowers on a wall at the Botanical Gardens in Christchurch, New Zealand. File/AP

Meanwhile, New Zealand police arrested a 19-year-old man over an "abhorrent" threat made this week against one of the Christchurch mosques targeted in a mass shooting last year.

As New Zealand prepares to mark the one-year anniversary of the massacre by a self-avowed white supremacist that killed 51 Muslim worshippers, police condemned the threat and increased patrols at two mosques that were attacked.

The intimidating message made on an encrypted messaging app reportedly showed a man in a balaclava sitting in a car outside the Al Noor mosque accompanied by threatening text and a gun emoji.

Police said they had increased patrols around the Al Noor and Linwood mosques, the two places targeted in the attack, and would be maintaining a visible presence in the community.

The patrols come after a new threat against the Al Noor mosque emerged this week, media reported.

The community flagged a picture of a man standing outside the mosque wearing a balaclava with a threat against worshippers this week, media said.

Police confirmed they had been notified of the image and said it had been referred to the chief censor for consideration as to whether it should be classified as objectionable material.