The attack on Manchester Arena that killed 22 people and injured 59 others was "abhorrent and vicious" says Prime Minister Bill English.

He offered his condolences to the victims and their families in a book delivered to Parliament by British High Commissioner Jonathan Sinclair on Wednesday, as details of the attack and its aftermath came to light.

British authorities have confirmed many children are among the dead in the bombing outside the arena where American pop star Ariana Grande had just finished performing on Monday night (UK time).

"I think all New Zealanders' hearts go out to the families who have lost their loved ones and to the nation because this is the last thing you would expect to happen when you send your children off," Mr English said.

In the book he expressed hope that the victims, many children including one as young as eight, rest in peace after the "abhorrent and vicious attack".

A line of government MPs followed Mr English to offer their condolences, including Foreign Minister Gerry Brownlee and Justice Minister Amy Adams.

Labour leader Andrew Little and his deputy Jacinda Ardern were also among the first to express their condolences.

"I said that to the people of Manchester and the United Kingdom that our thoughts are with them at this tragic and difficult time and that through solidarity and compassion we will get through these dark times and strive for a more peaceful and beautiful world," Mr Little said.

Meanwhile British police confirmed the attacker as 22-year-old Salman Ramadan Abedi, reportedly after his identity was leaked by US officials against the wishes of police and security services.

In response to the attack British Prime Minister Theresa May has lifted the United Kingdom's terror threat assessment to critical, indicating an attack is imminent.

Mr English said there had been no change to the terror threat level in New Zealand and that would only be done if there was indication of a threat here.

"We can assure (people) that the risks of an attack are monitored constantly, that we have a strong focus on public safety, that there's no evidence of heightened risk of attack in New Zealand, or around the Lions Tour but we remain vigilant," he said.

Increased security at stadiums hosting British and Irish Lions Tour matches will be discussed over the coming days, he said.

The condolence book will be open to the public to sign at the British High Commission in Wellington until Friday.

VIDEO BY FELIX MARWICK / NEWSTALK ZB