"It was not safe to run services because groups of youths were on the platforms, pushing and shoving," a spokesman said.
"There were 15 security guards on duty keeping two groups apart - the youths dispersed when police arrived."
A police spokeswoman said police were called to the scene to restore order, but no arrests were made.
Public Transport Users' Association coordinator Jon Reeves said that when he arrived at Britomart at about 11.45pm to catch a train home to South Auckland, there were more than a thousand people milling around the station's street level, not knowing how to get home while their access to underground platforms remained blocked.
"I have never seen such chaos," said Mr Reeves, who took a cellphone photo of a barricade still in place at 11.48pm.
"People were lost and it was busier than peak time during a week day. I talked to police and security and no-one knew what was going on.
"The worst thing was there was no-one from Auckland Transport - they had all the information displays and PA systems but there was no communication with passengers about what to do - it was just shocking."
Mr Reeves said he came across would-be passengers who had never used trains before, before he caught a lift home in a car at about 12.15pm with someone whose partner drove in to the city to collect them.
He said he tweeted a message to Auckland Transport and also posted his concern on Facebook but "nothing came back."
"So it seems like Auckland Transport are really good at working Monday to Friday, 9am to 5pm, and with situations like that there are no contingency plans."
Auckland Transport spokesman Mark Hannan rejected Mr Reeves' accusation, saying the security guards "work for us" and they had been busy keeping the youths apart.
"For safety reasons we had to hold the trains because these people were fighting right by the lines and jumping up on the gates - and they were throwing signs and things."
Even so, no other damage was caused, and he said PA messages and display boards were used to notify passengers of the delays.
Text messages were sent to regular trains users.
Mr Hannan said it was unfortunately not the first year trouble had overflowed to Britomart from Christmas at the Park "even though it's supposed to be an alcohol-free event."
He understood the youths had been skirmishing in other parts of downtown Auckland as well, although a police spokeswoman said no arrests were made.
The trouble followed disruption to about 3000 passengers during Thursday morning's commuter peak, when a new electric train was brought to a false emergency halt outside Britomart, blocking the line and forcing the termination of other services at Newmarket.
