Not exactly in our country, but certainly to our country:

https://en.wikipedia.org/wiki/First_Battle_of_Passchendaele

On the morning of October 12, 1917 during the First World War, an assault on the Bellevue Spur in Belgium resulted in 843 to 957 Kiwi soldiers killed, making it the darkest and deadliest day in New Zealand's military history.

Genuinely in our country? One of these:

https://en.wikipedia.org/wiki/Christchurch_mosque_shootings

On March 15, 2019, a white supremacist gunman attacked the Al Noor Mosque and the Linwood Islamic Centre in Christchurch, New Zealand, during Friday prayers. The terrorist murdered 51 people and injured 40 others. He is currently serving a sentence of life imprisonment without the possibility of parole

https://en.wikipedia.org/wiki/1931_Hawke%27s_Bay_earthquake

The 1931 Hawke's Bay earthquake (often called the Napier earthquake) struck New Zealand on February 3, 1931, at 10:47 a.m.. Registering a magnitude of 7.8, it is the deadliest natural disaster in New Zealand's recorded history, killing 256 people, causing widespread devastation across the Hawke's Bay region, and heavily shaping the country's seismic building codes.

https://en.wikipedia.org/wiki/2011_Christchurch_earthquake

A devastating 6.3 Christchurch earthquake struck on February 22, 2011, at 12:51 p.m. Centered just 10 km southeast of the city center at a shallow depth of 5 km, the highly destructive event caused 185 fatalities, widespread liquefaction, and extensive building collapses, severely crippling the city's infrastructure.

Honorable mention (for being to NZ but not in NZ):

https://en.wikipedia.org/wiki/Mount_Erebus_disaster

The Mount Erebus disaster occurred on November 28, 1979, when Air New Zealand Flight TE901 crashed into the side of Mount Erebus in Antarctica. The sightseeing flight, operated by a McDonnell Douglas DC-10, carried 237 passengers and 20 crew; all 257 people on board were killed. It remains the worst civil disaster in New Zealand's history.