Cairns was reported to be on life support following an aortic dissection.
McCullum’s message of support comes despite him having differences with Cairns in the past.
"I want nothing more to do with him" was McCullum’s remark after Cairns was cleared in 2015 of perjury charges related to match-fixing charges.
Speaking on his radio show, McCullum said their "relationship is unimportant" in the current situation.
"It's a difficult subject to obviously talk about. We haven't seen each other for quite a long time," he stated.
"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."
According to the reports, Chris Cairns suffered an aortic dissection in Canberra last week, which is a tear inside the body’s main artery.
"He's in a serious but stable condition in intensive care," a St Vincent's Hospital spokesman told AFP.
While Cairns' mother Sue is in Canberra currently, she is not being able to go to the Sydney hospital due to COVID-related restrictions.
"Our relationship is unimportant in the whole thing, the fact is that Chris is a father and also a son to Lance and Sue," McCullum said.
"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."
As soon as the news of his illness came out, social media was flooded with messages of support for Chris Cairns.
Chris Cairns represented New Zealand in 62 Tests, scoring 3320 runs and claiming 218 wickets including 5 centuries and 13 five-wicket hauls. He also featured in 215 ODIs, aggregating 4950 runs and taking 201 wickets. He was the Player-of-the-Match in Black Caps’ 2000 Champions Trophy final triumph over India.
Cairns’ last appearance for New Zealand was one of his only two T20I games against the West Indies at Auckland in 2006.
(With AFP inputs)