Social media companies have backed a new plan to stop violent terrorism spreading online, putting their names to the scheme with support from 48 countries at the United Nations.

Google and Facebook are among the companies that will overhaul a counter-terrorism centre to prevent a repeat of the Christchurch terrorist attack in March, which killed 51 people and wounded 49.

Sheryl Sandberg, with NZ Prime Minister Jacinda Ardern, acknowledged Facebook did not take down videos of the Christchurch killings quickly enough. Credit:AP

New Zealand Prime Minister Jacinda Ardern and French President Emmanuel Macron announced the agreement at the United Nations as they and other leaders prepared for the UN general assembly this week.

Prime Minister Scott Morrison joined the two leaders and others at a meeting in New York on Monday on strategic responses to violent extremism.