A policewoman patrols past a memorial wall outside the Linwood Mosque in Christchurch on Thursday, six days after the twin mosque shooting massacre that claimed the lives of 50 people. Photo: AFP

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.

"This type of imagery has no place in Aotearoa New Zealand - it is abhorrent and will not be tolerated," Canterbury police commander superintendent John Price said in a statement.

Price said the man had been charged on an unrelated matter and police were still gathering evidence over the mosque threat.

Price praised members of the public who reported the threatening image and warned against sharing it online.

He said the image had been referred to New Zealand's chief censor David Shanks to see if it should be classified as objectionable material.

Shanks last year outlawed video livestreamed on social media during the shooting and a Christchurch man who later shared it was jailed for six months.

The alleged mosque attacker Brenton Tarrant, an Australian national, is due to go on trial on June 2 facing terrorism charges plus 51 counts of murder and 40 of attempted murder over the killings.

AFP