New Zealand's prime minister has announced a top-level inquiry into the circumstances surrounding the massacre of 50 people in two Christchurch mosques.

Prime Minister Jacinda Ardern says the country's highest form of investigation, a royal commission of inquiry, was appropriate for "matters of the gravest public importance."

Her Cabinet had previously agreed on holding an inquiry, but had not decided what kind of investigation would be held.

She said the Cabinet agreed Monday a royal commission of inquiry "will look at what could have or should have been done to prevent the attack."

An Australian white supremacist has been charged with murder for the March 15 attacks.