New Zealand police said on Tuesday they were investigating a threat made this week against one of the Christchurch mosques where 51 Muslim worshippers were killed in a mass shooting a year ago.

Policemen stand gurard near a mosque in Christchurch, New Zealand, on March 17, 2019. The death toll from the terror attacks on two mosques in New Zealand's Christchurch has risen to 50. (Photo: Xinhua)

Amid plans to mark the one-year anniversary of the March 15 massacre, police said the threat to Christchurch's Al Noor mosque was posted this week on encrypted messaging app Telegram.

Reports said the message showed a man in a balaclava sitting in a car outside the mosque accompanied by threatening text and a gun emoji.

"We have strong leads that we're following up and I'm confident that we will establish who this person is," Canterbury police commander superintendent John Price told Radio New Zealand, adding, "We're very close." 

Al Noor was one of two mosques targeted by a self-avowed white supremacist last year in what Prime Minister Jacinda Ardern described as "an unprecedented act of terror [that] shattered our small country."

The timing of the latest threat comes at a sensitive time as planning is underway for a memorial service in Christchurch to mark the anniversary.

Details of the service at the city's Hagley Park have not yet been finalized.