Police in New Zealand said on Sunday that the right-wing terrorist behind a massacre at two mosques in Christchurch was acting alone, as it emerged the killer's manifesto had been sent to the country's prime minister minutes before the tragedy unfolded.

Australian Brenton Tarrant, 28, sent a racist, rambling manifesto to prime minister Jacinda Ardern in which he denied being linked to any organisations and said he was acting on his own.

The document, which praised President Donald Trump and Norwegian mass killer Anders Breivik, was emailed to Ms Ardern's office just 9 minutes before the attack began.

However, a senior White House official said it was unfair to cast the shooter as a supporter of President Trump based on one reference to him in the manifesto.

Acting chief of staff Mick Mulvaney told "Fox News Sunday" that the shooter was a "disturbed individual" and an "evil person." Mr Mulvaney said attempts to tie the shooter to any American politician "probably ignores some of the deeper difficulties that this sort of activity exposes."

It came as a man whose wife was killed in the attack as she rushed back into a mosque to rescue him said he harbours no hatred toward the gunman, insisting forgiveness is the best path forward. "I would say to him 'I love him as a person'," said Farid Ahmad, whose wife Husna Ahmad, 44, was killed at the Al Noor mosque - the first of two targeted by the gunman. "I could not accept what he did. What he did was a wrong thing," he added.