Police arrested two people in Christchurch and charged one of them after an online threat to Linwood Islamic Centre and Al Noor Mosque.
The arrests come less than two weeks out from the second anniversary of the terror attacks at the mosques in which 51 people were killed.
In a briefing after 8.30pm on Thursday, Canterbury district commander superintendent John Price said the threats were made on 4Chan. The post was tipped off to police through their Crimestoppers line a couple of days ago.
He said he understood the threat had been taken down now.
Price said one of those arrested had already been released and a 27-year-old man has been charged with threatening to kill.
"[The threat] was credible enough that we took action straight away."
As a result of the threat made and the context, both mosques were searched "pretty shortly afterwards".
What was found during the search cannot be released at this stage, police said.
The 27-year-old man is due to appear in Christchurch District Court on 5 March.
Police said they were not in a position to details on the nature of the threat as it was before the courts now.
Any threat made on people and the community was not tolerated, Price said.
"We take all threats of this nature seriously and we are working closely with our Muslim community.
"Any messages of hate or people wanting to cause harm in our community will not be tolerated - it's not the Kiwi way."
This behaviour needed to be called out, he said.
"I think we should all have eyes open and looking out for each other."
The Muslim community was spoken to "pretty soon" after police were made aware of the threat, he said.
The Armed Offenders Squad had also assisted as a precautionary measure.
Intensive investigation and inquiries were undertaken by a team fulltime following the threat and further charges were being considered.
"It is a criminal investigation, a threat has been made, and we have acted and responded with the appropriate charge," Price said.
All New Zealand police will be on alert during the Christchurch mosque attack memorial, with heightened visibility of police at the mosques already planned, he said.
"It's very concerning for me but also for New Zealanders as a whole," Price said.
He said New Zealand was at a medium threat level.
He hopes the Muslim community have trust and confidence in police to respond to threats like this.
In a statement after the arrests, Muslim Association of Canterbury spokesperson Abdigani Ali said they appreciated police acting promptly on the threat and in consultation with the association and Muslim leaders in the community.
"There is no place for hate rhetoric and hate crimes in our country and every community no matter their race or beliefs should feel a sense of safety and belonging."
In another statement, Islamic Women's Council of New Zealand (IWCNZ) said the threats were "especially cruel" as we approach the second anniversary of the 15 March attacks.
"It shows the need for a strong national security system, with clear leadership and direction working with communities."
The IWCNZ thanked police for taking quick action and the member of the public who alerted police to the threat.
"There can be no tolerance for direct threats to people or buildings, whether these are made online or offline. We ask the public to be vigilant in reporting any such threats to authorities.
"We pray that the Muslim community in New Zealand stays safe. We know this will be a difficult time for them."