The royal commission - the most powerful judicial inquiry available under New Zealand law - was supposed to report back on December 10 but after receiving more than 1,100 submissions, this has been extended to April 30.
The inquiry is investigating whether New Zealand's police and intelligence services could have prevented the March 15 attacks in which a lone gunman opened fire on two mosques in the city of Christchurch killing and wounding scores of Muslims.
Minister of Internal Affairs Tracey Martin said more time was needed "to hear submissions and consider information".
An Australian, Brenton Tarrant, will go on trial next year, charged with 51 counts of murder, 40 of attempted murder and engaging in a terrorist act.
When he was arrested soon after the shootings, police recovered several firearms which had been legally bought, prompting a government crackdown on firearms including a ban on military-style semi-automatic rifles.
Martin said the public "deserves to know what the relevant state sector agencies knew about the individual's activities before the attack, what, if anything, they did with that information, what measures agencies could have taken to prevent the attack, and what measures agencies should take to prevent such attacks in the future."
She said there was significant public interest in the investigation as evidenced by the number of submissions.
"The commission requested an extension as it will need considerably more time to fully consider these submissions given the complexity of information and materials received," Martin said.
The Muslim community had also asked for the reporting date to be extended, she said.
Source: Al Jazeera