Prime Minister Jacinda Ardern is expected to announce a ban on military-style semi-automatic weapons and tighter controls on gun ownership following a Cabinet meeting tomorrow that will focus entirely on the Christchurch mosque shootings.

Ardern has been firm that the country's gun laws will change following the attacks on two mosques on Friday in which 50 people were killed.

She is also expected to announce a review of any failings of security agencies in the lead-up to the attacks.

Australian man Brenton Tarrant has been charged with murder over the shootings of worshippers at mosques in Linwood and Riccarton.

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Ardern indicated she had the support of Deputy Prime Minister Winston Peters for the changes, despite previous strong opposition from New Zealand First on the issue of tighter gun control.

"As I've already indicated, there will be changes to our gun laws. We will be discussing the more detailed policy at Cabinet tomorrow. Of course I've had conversations directly with the Deputy Prime Minister, as you would expect," Ardern told reporters today.

She was careful to reiterate that any legislative changes would take some time.

Ardern said security agencies would update Cabinet ministers tomorrow and those agencies would also be discussed.

"We need to make sure that we are looking more broadly at the work of our agencies, not just our intelligence services, police and our borders to ensure that we've taken a comprehensive approach and done everything we can to prevent any kind of action like this in the future."

She said there needed to be a "comprehensive response" to an range of issues the attacks had raised.

"I don't want to pre-empt that but I will be looking to move as swiftly on those areas where we need to respond as I can."

Opposition leader Simon Bridges said the National Party would back gun law reform.

"We will be ready and prepared to be constructive and to look at anything here because we do need to see some change," he said.

Tarrant emailed a copy of his hate-filled manifesto, which included his intended targets, to the Prime Minister's office and others, including media, just minutes before carrying out the attacks which he livestreamed on Facebook.

He obtained a New Zealand gun licence in November 2017 and legally bought the weapons he began purchasing a year later.

He was reportedly a member of a rifle club in Dunedin and practised shooting an AR-15 semi-automatic rifle there. It was an AR-15 modified to be fully automatic that was thought to have been used in the shootings.

At present it is the gun owner that is licensed rather than the weapons themselves.

Ardern would not be drawn on calls for an immediate moratorium on the sales, imports and advertising of semi-automatic weapons following media reports that people were "panic buying" ahead of any ban.

"I have heard and seen reports, but as yet unverified, of potential activity but as yet I haven't received official advice as to whether or not there has been any change in purchases. That's something I have asked for follow up information," she said.

Otago University public health experts Marie Russell and Hera Cook said now was the time for the gun lobby to stand aside and a moratorium to be immediately implemented.

"Along with countries in North America, New Zealand is one of the few places where owners don't have to register each weapon, except for some limited categories of firearms," Cooks and Russell said in a statement.

"It is likely that some firearms enthusiasts are expecting a ban or restrictions to be imposed and will be racing to purchase these items."

Gun City owner David Tipple is expected to comment tomorrow. Gun City bills itself as the world's largest gun store and sells a range of military style semi-automatic weapons.

Meanwhile, Parliament will resume on Tuesday only long enough for parliamentarians to pay tribute to the victims of the mosque attacks before rising for the day.

All public tours of Parliament have been cancelled and the electorate offices of MPs from Canterbury to Dunedin will be closed.

Condolence books will be opened at Parliament and the National Library for the public to leave messages.