New Zealand moved to ban military-style semiautomatic weapons, assault rifles and high-capacity magazines, stepping up its response to Friday’s slaughter of 50 worshipers at two Christchurch mosques by a lone gunman.

Shooter Brenton Tarrant held a valid gun license, but the weapons used in the terrorist attack included semiautomatic rifles that had been illegally upgraded with higher-capacity magazines to enable 30 bullets or more to be fired without reloading. His license restricted his magazine capacity to seven bullets....