"Four men" attacked Ryder as he left Aikman's Bar in Merivale shortly after midnight (around 12.30 a.m. local) early Thursday after having "a few drinks" with Wellington Firebirds teammates, The New Zealand Herald reported.
The "brutal assault" has left Ryder in an induced coma with a fractured skull and multiple injuries.
The New Zealand team's official Twitter page said: "The whole NZC team's thoughts are with (Jesse) this morning."
Former Australian pacer Damien Fleming wrote on twitter: "Our thoughts to Jesse Ryder in intensive care after being assaulted. Shocking news."
The left-hand batsman, who has been involved in alcohol-related incidents in the past, was due to leave for India to represent Delhi Daredevils in the Indian Premier League beginning April 3.
IPL CEO Sundar Raman wrote on twitter: "Not the best cricketing news to wake up to. Jesse Ryder get well soon."
Cricket commentator Harsha Bhogle said: "Oh dear, it looks really bad for jesse ryder." He added: "my first thoughts on reading the jesse ryder news were about david hookes; a valued colleague and a friend. bars are so so over-rated."
West Indies' explosive opener Chris Gayle tweeted: "Will have Jesse in my prayer. Pray for Jesse. Let's hope he gets better soon."
Former England captain Michael Vaughan posted a Twitter message: "Devastating news regarding Jesse Ryder. Just hope he can pull through. Thoughts are with him and his family."
Australian legend Glenn McGrath tweeted: "Our thoughts are with NZ cricketer Jesse Ryder & his family. Jesse was assaulted & is in intensive care. We wish him a speedy & full recovery."
"Just arrived in Sydney to the news about Jesse Ryder! C'mon big man hang tough," his fellow Kiwi Scott Styris added.
England off-spinner Graeme Swann tweeted: "Shocking news about Jesse Ryder, fingers crossed for a full and speedy recovery for the big man."