Police said they were still working to ascertain how the incident unfolded - and where it began to play out.
The owners of Club Envy have been contacted but are yet to comment.
A man who was in the nightclub said members of a gang had been initimidating and eyeballing people in the lead-up to the stabbings.
Police are also in attendance at a nearby McDonalds and a third address.
"Police were called to a disorder incident on Market St at about 2.55am, one person died at the scene," Blenheim Area Commander Inspector Simon Feltham said.
"Two people were also seriously injured and taken to hospital where they remain in a serious condition.
"Police are speaking to several people in relation to this incident and an investigation is under way."
Just before midday police asked for anyone with footage of the night - anything they thought was related to the attack - to pass it on.
"Police are encouraging anyone with video footage from the incident to upload it using the following link: https://alaia.nzpolice.org/," said a spokesperson.
They are expected to provide a further update later today.
A person who had been at the bar told the NZ Herald and Newstalk ZB: "It's not the normal thing that happens on a night out in Blenheim.
"There was a large presence of motorcycle gang members ... I heard they were Rebels.
"I left there about 2.55am and they were closing up, I'm not really sure what happened but heard a scuffle broke out and knives came out and people got stabbed.
"One of them died ... and they got chased all over town by police ... yeah, it was a crazy morning in Blenheim."
The witness said the nightclub was "usually fine" and there was not much of a gang presence in Blenheim.
He said it was "shocking".
He had been back to the scene after the incident and saw people on stretchers, a police tent and emergency services.
"It was a very bit shock ... the Armed Offenders Squad ended up being called ... it was all on," he said.
"It's a bit rough, I'm not used to anything like this happening."
Feltham said police were conducting a number of scene examinations today.
The Market St area was cordoned off and officers could be seen working beyond the police tape.
"The scenes remain cordoned off and there will be a police presence in the area as we work to understand the circumstances of these incidents and ensure the community's safety," he said.
"Anyone who was in the area at the time and may have seen anything that could assist is asked to get in touch. Police would like to hear from anyone who may have footage of the incidents."
Police asked those who can help to phone them on the 105 non-emergency line and quote job number P046918039. Alternatively, they said, information can be provided anonymously by phoning CrimeStoppers on 0800 555 111.
Feltham said further information will be released when it becomes available.
Sociologist and gang expert Dr Jarrod Gilbert said it was unusual for gangs to kick off in the Blenheim area.
"There's never been any huge drama up there," he said.
Historically the Lone Legion gang were the most present, but more recently most members had "patched over" to the Outlaws MC.
In 2007 Carl MacDonald was murdered by a patched member Lone Legion member Aaron Harvey.
Harvey gunned MacDonald down with a sawnoff .22 calibre rifle after a confrontation outside the gang's headquarters.
MacDonald had confronted Harvey for throwing a brick at his car.
Other than the murder, there has been little gang action or inter-gang warfare in the area.
More to come.
