WELLINGTON (AFP) - New Zealand police found a suspected bomb and ammunition on Tuesday (April 30) in a vacant lot in Christchurch, the scene of last month's mosque shootings which killed 50 Muslim worshippers.

Police cordoned off streets late on Tuesday afternoon in the Phillipstown area just outside the central business district and evacuated local houses.

Shortly after 6pm local time they said a military bomb squad had rendered a suspicious package safe and a 33-year-old man had been arrested and was "assisting police with inquiries".

"Police have located a package containing a suspected explosive device and ammunition at a vacant address on Newcastle Street in Christchurch," superintendent John Price said in a statement.

"The NZDF EOD (NZ Defence Force Explosive Ordnance Disposal) team have rendered the package safe." Price did not provide any background about the man or his possible motivations.

Christchurch remains on alert after the March attacks in which a self-styled white supremacist shot dead 50 people and injured another 39 in attacks on two mosques.

Just last week, the South Island city hosted a massive security operation as Britain's Prince William visited survivors of the atrocity and praised the country's response to the massacre.

While the cordons prompted by the latest scare had been lifted, Price urged residents to remain vigilant.

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"Members of the community are asked to continue to report any concerning behaviour or activity in their neighbourhoods as we all work together to ensure the safety of our community," he said.

The gunman allegedly behind the mosque shootings, 28-year-old Australian Brenton Tarrant, is in a maximum-security prison in Auckland has been ordered to undergo psychiatric tests.