"We know that winter is a hard time ... so we prepare."
"We make sure we have got good supplies of food parcels, we do winter drives and we make sure that we've got really good stocks of blankets and bedding."
Many people without homes were simply unable to cope alone once temperatures got this cold, Dame Robertson said.
"We've got an increased number of people who are coming in really just to get warm who have been on the streets over night or sleeping in cars.
"People who maybe coped before when it wasn't too cold are now seeking extra equipment, some are now seeking housing."
As well as providing extra supplies, City Mission also rostered on more staff to deal with the increase of people coming in with winter coughs and colds.
Dame Robertson said the Mission worked hard to meet everyone's needs.
"We have a policy of not turning people away so we do meet the demand.
"That's not to say that it's easy but we do it."
The week's cold snap may be on the way out but people's needs would not go with it, Dame Robertson said.
"We've already started the winter demand and the cold snap is just one piece of the whole winter puzzle. It is generally colder, it is generally wetter, people are sicker."
Down in Wellington, City Mission CEO Michelle Branney said numbers of people seeking help "absolutely" spiked every winter.
"The cold weather has definitely meant at the drop in centre we've had more people coming in wanting a hot meal."
Ms Branney said the cold snap was just part of an increase in demand.
"Every June, every July [numbers rise] ... It's all cold. It's been building up to this."
Numbers of people seeking a hot meal were increasing steadily as the months got colder.
So far this June 102 people have been fed per day on average at Wellington's City Mission compared with 87 in May and 74 in April.
Further south still, the latest big freeze left Christchurch's post-earthquake homeless population in a rut - not just rough sleepers but people dossing in garages, cars, and broken houses.
Christchurch City Mission's night shelter, which could house seven women and 30 men, was at full capacity on Monday night, NZME. reported yesterday.
The Mission put on an extra women's bed because of the cold nights and the Salvation Army was handing out extra blankets and heaters.