Police are investigating after a threat to Christchurch mosques was made online.

Photos posted on a website on Monday morning mentioned "doing something like what happened on March 15 last year".

The encrypted messaging app has been likened to other popular websites which attract far-right extremists.

Islamic Women's Council of New Zealand Anjum Rahman said they have alerted community leaders and asked police to increase security around mosques.

Rahman said it's not the first threat they've received.

A police spokesperson said they are aware of the matter and take all threats to the community extremely seriously.

Inquiries are ongoing.

"For operational reasons we will not be providing details on security matters ahead of 15 March 2020," police said.

"Police are working closely with the organisations involved and we have the common aim of ensuring the safety of our communities."