New Zealand's opposition party says a Royal Commission is the only way to independently know if the New Zealand mosque attack could have been prevented.
National Party leader Simon Bridges on Sunday called for an investigation into security and intelligence agencies and security legislation.
"We need to understand whether this could have been prevented," he said.
"It will need to ask hard questions about whether our security and intelligence agencies had their focus in the right places."
Fifty people died when a lone gunman opened fire in Christchurch's Masjid al Noor and Linwood Masjid, and another 50 were injured.
Mr Bridges said a project to scan internet traffic, abandoned in 2013 by the then National government, would have "given an extra degree of protection to all New Zealanders".
He was a cabinet minister at the time under prime minister John Key.
The line between privacy and security must now be reconsidered, he said.
The proposal has the support of ACT leader David Seymour, who said terms of reference should include why the alleged gunman was not detected by authorities and whether intelligence services are adequately resourced.
"These terrorist attacks are a clear reflection that our intelligence services did not work to protect New Zealand in the way they should have," he said.
Mr Seymour, who is his party's sole MP, has accused the government of rushing the response, and could thwart Prime Minister Jacinda Ardern's hopes for a unanimous support for gun law changes.
"By forcing gun law reform through in three weeks the government will ensure there is no real opportunity for New Zealanders to have their voices heard," he said on Friday.
"It is deeply disappointing that the government is responding to the Christchurch terrorist attack by abandoning proper democratic process."
From Thursday afternoon weapons like those allegedly used by 28-year-old Australian Brenton Harrison Tarrant in the attack became illegal under interim measures.
Legislation is expected to be introduced by April 11.