Dozens of Christchurch gun owners have handed over their weapons in exchange for cash in the first of more than 250 planned buyback events around New Zealand after the government outlawed many types of semi-automatics.

Police said they paid more than $NZ200,000 in total to 68 gun owners in the first few hours of the event held on Saturday.

In April, New Zealand lawmakers rushed through new legislation to ban military-style weapons after 51 people were killed at two Christchurch mosques on 15 March.

Brenton Tarrant, an Australian, is facing 51 charges of murder, 40 of attempted murder and one of engaging in a terrorist act in relation to the Christchurch shootings, and has pleaded not guilty to all charges. The trial, estimated to take six weeks, will go ahead on 4 May 2020.

A bill outlawing most automatic and semi-automatic weapons, and components that modify existing weapons, was passed by a vote of 119 to 1 in April in the House of Representatives after an accelerated process of debate and public submission.

The prime minister, Jacinda Ardern, had spoken emotionally during the bill’s final reading, telling of the traumatic injuries to the victims of the March attacks, whom she had visited in hospital.

“I struggle to recall any single gunshot wounds,” Ardern said. “In every case they spoke of multiple injuries, multiple debilitating injuries that deemed it impossible for them to recover in days, let alone weeks. They will carry disabilities for a lifetime, and that’s before you consider the psychological impact. We are here for them.

“I could not fathom how weapons that could cause such destruction and large-scale death could be obtained legally in this country.”

The government has set aside more than NZ$200m to buy back weapons such as AR-15 style rifles, although many gun owners remain unhappy with the compensation on offer.

Under an amnesty, gun owners have until December to turn over their now-banned weapons.

The total number of firearms in New Zealand is estimated to be between 1.2m and 1.5m.