Black Caps great Brendon McCullum put aside past differences to lead messages of support Wednesday for former New Zealand star Chris Cairns, who is fighting for his life in a Sydney hospital.

Cairns, 51, one of the world's top all-rounders in the early 2000s, was transferred to a specialist unit at St Vincent's Hospital after failing to respond to treatment after a serious cardiac problem in Canberra last week. 

McCullum had once said "I want nothing more to do with him," after his former teammate was cleared in 2015 of perjury charges related to match-fixing allegations.

McCullum was the star prosecution witness in the high-profile trial but said Wednesday 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," McCullum said on his radio 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."

Cairns was reported to be on life support following an aortic dissection, which is a tear in the inner layer of the body's main artery.

"He's in a serious but stable condition in intensive care," a St Vincent's Hospital spokesman told AFP.

Cairns' mother Sue is in Canberra but unable to get to his Sydney hospital bedside due to Covid-19 travel restrictions in Australia.

"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."

New Zealand Cricket has declined to comment on his illness, citing respect for Cairns' family privacy.

Social media was flooded with messages of support, including from Indian batting great VVS Laxman and the Nottinghamshire Cricket Club where Cairns also played.

England's Barmy Army supporters club tweeted: "Sending huge best wishes to @BlacksCaps legend Chris Cairns who is on life support in Australia. Pull through champ."

Cairns, whose father Lance also played for New Zealand, at one time held the record for the most sixes in Test cricket -- a record now held by McCullum -- and was the sixth player to achieve the all-rounders double of 200 wickets and 3,000 runs.