The spare sheet of paper upon which Jacinda Ardern hurriedly jotted her first thoughts following the Christchurch attacks has revealed a defining phrase heard repeatedly since: "they are us".

In an interview with the UK newspaper The Guardian, Ardern released an A4 sheet on which she scrawled her initial notes for a speech at Devon Hotel in New Plymouth at 4.20pm, March 15.

A few short sentences on it were highlighted by the Prime Minister at the time:

"One person custody may be other offndr.
Act of exraordnry violence. It has no place in NZ.
They are us."

That last phrase now opens the official book of condolences for the victims of the Christchurch in Parliament's Grand Hall in Wellington.

"On behalf of all New Zealanders we grieve, together we are one, they are us," the Prime Minister's message opens the book with.

It also features prominently in the celebrated first speech the Prime Minister gave to the media in New Plymouth on March 15.

"Many of those who will have been directly affected by this shooting may be migrants to New Zealand, they may even be refugees here. They have chosen to make New Zealand their home, and it is their home," Ardern said.

"They are us. The person who has perpetuated this violence against us is not. They have no place in New Zealand. There is no place in New Zealand for such acts of extreme and unprecedented violence, which it is clear this act was."

An inversion of the phrase also appeared in subsequent addresses Ardern gave on the gunman himself.

"You may have chosen us - we utterly reject and condemn you", Ardern said of the attacker.

The first 72-hours of the Prime Minister's schedule following the Christchurch attacks have been detailed by the Herald.

In the Guardian interview, published today, Ardern also revealed she borrowed the headscarf she wore addressing members of the Muslim community in Christchurch on March 16.

"When I had the all-clear to go down [to Christchurch] on Saturday, I asked a friend if they had something for me to borrow," Ardern said.

"If I'd been [at home] in Auckland it would have been different, but I didn't have scarves with me. So I asked if she had something I could borrow, because for me it was just a mark of respect. It was naturally what you would do. So, no, I didn't really think about that, either."

Ardern also touches on the Christchurch attacks not having diminished the optimistic outlook she has always sought to guide her politics by.

"No. My belief in the humanity of New Zealanders has strengthened. I just know we have a lot of work to do to make that universal," she said.

"I am an optimist. I was born one and politics has not beaten it out of me yet.

"People have remarked upon the way we've responded, but to me there was no question. You need to remove some of the politics sometimes and just think about humanity. That's all."