New Zealand Police said Ryder, 28, had been rushed to Christchurch Hospital after suffering the injuries early on Thursday morning and remained there in a critical condition.
"It appears that Jesse has been the victim of a serious assault and suffered head injuries as a result," Detective Senior Sergeant Brian Archer told a nationally televised news conference.
"Jesse was taken to hospital where he remains in intensive care in a critical condition in an induced coma after suffering multiple injuries."
Ryder reportedly suffered a fractured skull and a collapsed lung, though it is not yet known if the injuries are life-threatening.
Ryder, who had been drinking at the bar with team-mates from Wellington, had been involved in two incidents, the first outside a bar with three other people.
Two of the three then followed him across the road to a nearby fast food restaurant, where one of them apparently assaulted the cricketer.
Local media reports have stated that Ryder was hit from behind, though the police did not confirm this.
Police are examining closed-circuit video footage, and "at least 10 or more witnesses" apparently saw the attack, according to Archer.
"At this stage we have not identified the people involved in the incident but are following positive lines of inquiry," he added.
"We are asking for witnesses to come forward and to speak to us about it."
