The message written on the footpath outside Masjid An-Nur in Riccarton yesterday, and was reported to the police by the Muslim Association of Canterbury.
RNZ understands the message was written while worshippers were at afternoon prayers.
Canterbury police District Commander John Price said the incident is being taken very seriously.
"We take any messaging that has any concern to it extremely seriously and a report has been completed and that has been assigned to an investigator," he said.
Superintendent Price said the timing of the message - just days after the Royal Commission into the 2019 terrorist attack was released - was worrying.
"No matter what time it it is, any messaging that has an undertone that is not right for us in New Zealand is concerning," he said.
"Whether or not it would be on the back of the Royal Commission of Inquiry or not it's irrelevant. At the end of the day, it's about making sure we get to the bottom of what's actually happened, why it's happened and who has done it."
The police have a greater visibility around Masjid-An-Nur to provide reassurance to the community.
Masjid An-Nur was one of the two mosques targeted during the 15 March 2019 terror attack.
Forty-four people were murdered at the mosque, with another seven people murdered at the Linwood Islamic Centre.