Some of the world's biggest investors have sent an open letter to Facebook, Twitter and Google's parent Alphabet in a bid to put more pressure on the companies to prevent the sharing of objectionable content across social media.

It's just over a year since the Christchurch terror attack which killed 51 people in two mosques on March 15 with the content live-streamed and shared widely on the internet.

Soon after the event the $37 billion New Zealand Superannuation Fund called on other big investors to join it in pressuring the major social media companies to stop allowing the live-streaming of objectionable content.

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Since then social media companies have made some tweaks but the investors say it has not been enough.

It an open letter published today the group who represent US$7.5 trillion in assets said they had been "dissatisfied with the response from your senior executives and boards".

"The failure to respond to these actions creates a significant business risk, beyond the harm caused to the global community. You have a duty to address that," the letter stated.

The investors said as owners of the companies they welcome changes made to the platforms but said mass shootings continued to be disseminated across the platforms.

"They remain open to abuse.

"It is time that your respective organisations acknowledge the influence you have within society and embrace the responsibility that comes with this."

The investors said they wanted clear lines of governance and accountability from senior executives and board members to ensure social media platforms can not be used to promote objectionable content and sufficient resources dedicated to combating the livestreaming and spread of objectionable material across the platforms.

They also said legislation that governs these companies needed to be to modernised to protect the public from exposure to similar content in the future.

Matt Whineray, chief executive of the New Zealand Superannuation Fund. Photo / File
Matt Whineray, chief executive of the New Zealand Superannuation Fund. Photo / File

New Zealand Super Fund chief executive Matt Whineray said the Christchurch terror attack was a horrific and tragic event.

"We have made our voice known to these companies since the attack in Christchurch.

"While some positive changes have been made, more needs to be done at the executive and board level to build accountability and ensure these platforms cannot be used to spread objectionable material."