"It is clearly a very thoroughly planned and orchestrated attack and my heart goes out to those who were in the mosque, who died, who are badly injured, their families and those that are grieving."
Clark said the massacre, that has claimed forty-nine lives, is something that New Zealand has "never ever seen".
"We have to go back 29 years to Aramoana mass shooting but this is something that we have never ever seen - in a sense that it shatters our illusions that we are safe," she said.
"New Zealand is in shock that the things we see on TV from other parts of the world, could happen in peaceful Christchurch.
Police stand outside a mosque in central Christchurch where 49 people were killed in a mass shooting. Photo / Mark Baker
"But wherever these extremist ideas are present, it is always possible that something like this can happen.
"Norway, for example, experienced it with 77 dead several years ago with such an extremist who killed many young people at a summer camp on an island. This is our moment like that."
Clark said the incident would call for "deep reflection".
"There are many unanswered questions in people's minds, but right now the key issue is that we all grieve with the people who have been personally, deeply and tragically hurt by this.
"It is also very heartening to see the solidarity coming from overseas. It is one of those situations where everyone stands with New Zealand," she said.
Very shocked to hear of shooting & many deaths & injuries at Christchurch NZ mosque 🕌 where people were at Friday prayers. My heart goes out to all bereaved families & the Muslim community. This is a heinous hate crime. RIP. https://t.co/JtSVJeKI98
Clark said the devastating crime will be "condemned without reservation".
"The crime will be dealt with the heaviest force of the law that New Zealand can muster."
She said a whole life sentence would be on the cards.
"For an extremely serious crime like this life could be served. It is not a case of a homicide of a lower degree. This is an unprecedented terrorist attack in New Zealand."
Clark said there will also be a much higher degree of alert.
"It is horrible that we even have to think like this but I would think that every mosque will now be looking at security and maybe others from other faiths will also feel very vulnerable."
Clark also hopes the Government will take a look at gun laws in New Zealand as a result of the incident.
"I do personally hope that NZ Parliament will take a look at the gun laws because while we do have gun control, we can do better," she said.
"I think for parliament to come together now and show that a small western country can grapple decisively with this, will be extremely important."
