A review of the Kaikōura earthquake has revealed it generated an almost 7m tsunami and it was only luck that prevented it from causing more destruction.

The Ministry of Civil Defence review found the tsunami generated by November’s quake would have caused more damage if it had not happened at low tide and the land had not been simultaneously lifted upwards.

The largest wave was 6.9m in the Goose Bay area, just south of Kaikōura.

Several other areas had 3 to 4m waves.

Initially, no tsunami warning was issued but 40 minutes later a change in sea level was recorded. This prompted a warning to be issued an hour after the quake struck.

The way the warnings were issued drew criticism.

GNS natural hazards director Gill Jolly told Morning Report the tsunami could have been devastating and it was “very lucky” the quake happened at low tide.

She said initially the earthquake was rated as magnitude-6.8 and centred on land which did not pass the threshold to trigger a tsunami warning.

The quake was ultimately measured at magnitude-7.8 and was centred offshore.

“The complexity of the earthquake really became apparent over the next few days, weeks.

“Even now we’re trying to unravel the pieces of the jigsaw to understand what happened six months later.”

She said there would always be uncertainty in the early stages after an earthquake and she was proud of the way GNS staff handled it.

Civil Defence director Sarah Stuart-Black said the situation changed rapidly on the night and decisions were made with the information at hand.

She said civil defence was working with GNS to refine warning procedures but it was a fine balance between crying wolf and running the risk of not telling people to evacuate when it was necessary.

The threshold for triggering a tsunami warning has since been lowered from magnitude-7.5 to 7, Ms Stuart-Black said.

It was critical for people to know was that there would be no time to issue a warning if a large quake was centred near New Zealand, she said.

“If it’s longer than a minute, if it’s too hard to stand up, immediately evacuate inland, don’t wait for that official warning to take action to keep yourself safe.”