Sir John Key says February 22, 2011, was his toughest day as New Zealand Prime Minister.
The day after the Christchurch earthquake, Sir John described the quake as "death and destruction on a dreadful scale".
"New Zealanders have woken to a tragedy unfolding in the great city of Christchurch," he told reporters.
Speaking to The AM Show on Monday morning, Sir John reflected on that day 10 years ago where 185 people were killed.
"You want to help people and you want to do everything you can, and you want to do it now and that's not always possible because, initially, there are all sorts of things you're having to deal with," Sir John recalled.
He reflected on what he saw in Latimer Square just hours after the earthquake struck the city.
"The thing was covered in smoke - the news media had set up there and I tried to wander down as close to the CTV building as I could and you could see the carnage that was taking place there."
Sir John told host Duncan Garner by then he had been told about the number of fatalities. That gave him a sense of what the city was facing, he said.
"I walked in [to the emergency centre] - and this thing was shaking - the noise was just incredibly frightening.
"The police commander had said to me, 'look, there are at least 65 people dead, probably quite a lot more.'
Speaking of how the city had moved forward, Sir John said a lot of good progress had been made but there were still two things that hadn't progressed - one being the Christ Church Cathedral and the other a new stadium.
"I do think it's [a stadium] an important part of the DNA of Christchurch."
Work on the Christ Church Cathedral is finally starting to progress. In December, a 13 metre high and 18-tonne steel frame was craned into the front of the iconic landmark - described as a huge moment for the city.
Meanwhile, Christchurch City Mayor Lianne Dalziel said Monday's anniversary service will be a sombre affair.
She said the 10 years that followed the quake have taken a toll on the city.
"If expectations aren't fulfilled then hopes are dashed, and it takes a long time to recover from that - people say that the disaster is what happens afterwards."
Prime Minister Jacinda Ardern, who was a backbench Opposition MP at the time of the quake, said she had hoped people wouldn't have ongoing trauma as a result of the city's rebuild.
"The issues that we have to acknowledge there have been with Southern Response … people caught in disputes trying to seek resolution over the repair of their homes. No-one would have wanted to see that long trauma people have experienced," she told The AM Show.
"I think everyone, regardless of [their] political party, would acknowledge that's been hard for people."