It's understood Cairns (51) collapsed, having suffered a massive heart attack last week in Canberra - resulting in an aortic dissection, a tear in the inner layer of the body's main artery.
He has had an operation on his heart overnight in Sydney is recovering in specialised care, NZME understands.
According to a Newshub report, he has undergone several operations while in hospital, but has not responded to treatment as hoped.
McCullum testified against his former teammate in the 2015 perjury trial at Southwark Crown Court in London, during which Cairns was found not guilty of charges relating to alleged match-fixing.
"It's a difficult subject to obviously talk about. We haven't seen each other for quite a long time," McCullum said on his SENZ Breakfast show.
"We've been reflecting on just how fine a cricketer he was when the news came through and what he did for the game and New Zealand cricket throughout his career as well. Our relationship is unimportant in the whole thing, the fact is that Chris is a father and also a son to Lance and Sue. They've already had such tragedy in their life with the loss of Chris' sister a long time ago as well.
"It's a really difficult time for those people and I know the cricket community and all those who support the Cairns family will be suffering right now. Today my family and myself are thinking of those people who are suffering."
"I'm sure [Cairns] will get on with his life, and I'll get on with mine, and let's hope they never cross paths," McCullum said at the time.
Cairns' mother is unable to get to his bedside due to Covid travel restrictions.
Sue Wilson is in Canberra and will not be able to get to her son at a hospital he was transferred to in Sydney.
The city is currently under strict Covid-19 restrictions as it continues to deal with the latest Covid outbreak in the community.
Cairns is then said to have suffered an infection and had to be transferred to Sydney for hospital treatment.
Cairns the son of Black Cap legend Lance Cairns, was a right-hand batsman and fast-medium bowler. He was known as one of the finest all-rounders of his generation.
The Cairns family has endured personal tragedy. Chris Cairns' sister Louise was killed in 1993 when a cement truck failed to stop at a level crossing at Rolleston in the South Island and collided with the Southerner passenger train.
Former Black Caps teammate Andre Adams said on social media his thoughts and prayers are with Cairns and his family.
Fans were quick to share their support for Cairns, with many wishing him a speedy recovery and reminiscing about his memorable moments for New Zealand.
"Oh no. This is awful. Chris Cairns is responsible for one of my most treasured cricket memories: NZ v RSA at the Gabba for my 15th birthday.
"We were on the brink of defeat and he blasted an unbeaten century. And I admire the work he did for rail safety after his sister died," one wrote.
Australian journalist Brendan Bradford added: "Ah man this is awful. Chris Cairns was an absolute icon when I was growing up. A genuine superstar. Hope he pulls through this."
NRL journalist Brad Walter wrote: "Terrible news. I used to love watching Chris Cairns bat and bowl. He was the Kiwi version of Gary Gilmour."
