The 28-year-old was in an induced coma with a fractured skull and serious internal injuries, and his family were at his bedside.
The attack in the South Island city came just days before the talented batsman was to take up a lucrative Indian Premier League (IPL) contract.
Ryder has a history of disciplinary lapses and alcohol-related incidents, but although he had been drinking before the early morning assault, police said alcohol was not a factor.
Detective senior sergeant Brian Archer said there was a brief altercation involving Ryder and “two or three” people when he stepped outside the bar.
Ryder then crossed the road to a nearby fast-food outlet where he was attacked.
“The hospital advise he is in a critical condition. He is in an induced coma as a result of suffering multiple injuries,” Archer said.
Police believed there were at least 10 witnesses, one of whom, Emma-Louise Ferguson, told the New Zealand Herald it was “unbelievably brutal”.
“They had him on the ground and they were kicking him hard,” she said, adding: “It was literally right outside the door of the bar.
“When he was on the ground, I couldn’t actually see him but I could hear the they were kicking him as hard as they could.”
Media reports said the injuries included a punctured lung.
Prime Minister John Key, who led messages of support for Ryder, said the attack appeared “somewhat unprovoked, so there must be something behind that and the assault seems extremely vicious”.
“Obviously we wish him a speedy recovery,” he added.
A witness, identified only as Adam, told Fairfax Media that Ryder tried to fight back as four men “absolutely smashed him”.
He said he was not sure if the attackers were known to Ryder but said they “damn knew who he was”.
Police were viewing closed-circuit television coverage of the attack and Archer said they were following “positive lines of enquiry”.
The incident occurred while Ryder was having a night out with his Wellington team-mates after losing a season-ending one-day match to Canterbury.
The batsman is on a self-imposed break from international cricket to address “personal issues” following a series of incidents.
New Zealand Cricket Players’ Association chief executive Heath Mills said Ryder was known to have resumed drinking in recent weeks.
“It will be an ongoing struggle for Jesse and we need to do all we can to help him,” Mills said.
“What I will say is Jesse was having a few drinks with his team-mates at the conclusion of his season. The actual assault was not an alcohol-fuelled incident.”
Ryder, who last played for New Zealand a year ago, was due to fly to India this weekend to compete for the Delhi Daredevils, who paid US$260,000 for his services at an auction.
Last year, Ryder, who has a Test average of 40.93 and a highest Test score of 201, took his manager and a psychologist with him to India when he played for the Pune Warriors.
“We are all shocked by what has occurred and extremely concerned for Jesse. New Zealand Cricket’s thoughts are with him and his family,” said New Zealand Cricket chief executive David White.
In 2004, former Australian batsman David Hookes died from head injuries suffered in a fight outside a bar in Melbourne.