New Zealand PM Jacinda Ardern confessed her cabinet had backed gun law changes “in principle”.
Australian Brenton Tarrant, 28, a self-described white supremacist, has been charged with murder.
Police say the killer used military-style assault weapons modified to make them more deadly for the attack – all of which is legal under current laws.
No specific details were given by the prime minister at her press conference on Monday, but she said they would made clear soon.
“This ultimately means that within 10 days of this horrific act of terrorism we will have announced reforms which will, I believe, make our community safer,” she said.
Ms Ardern was appearing alongside her coalition partner and Deputy PM Winston Peters, who has previously opposed changes.
He said he fully supported the prime minister on the issue, adding: “The reality is that after one PM on Friday, our world changed forever and so will our laws.”
Ms Ardern said: “We have made a decision as a cabinet, we are unified.”
She also announced that an inquiry would look into the lead-up to the attacks, and what might have been done differently.
At the weekend, Ms Ardern said the suspect had a gun licence, obtained in November 2017, and owned five guns.