The NTEU NSW division received an audit log under freedom of information laws, which it said showed UON added a policy at 12.13pm on December 6 to remove emails from the NTEU domain from its permitted senders list.

It said a policy was added at 1.10pm the same day to flag emails from the NTEU domain as spam, meaning they were all held. This policy was removed at 2.43pm on December 9.

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NTEU Newcastle branch president Associate Professor Terry Summers said the logs "look really dodgy".

"I'm extraordinarily disappointed," Dr Summers said.

He said the union understood its emails had been "whitelisted" prior to December 6 "so people could have access to information about bargaining".

"There is absolutely no doubt from the [audit log] that there was an explicit blacklisting of the NTEU domain that is different from the spam filter just operating like normal," he said.

"The Vice Chancellor [Professor Alex Zelinsky AO] said he didn't do that, somebody did though. I don't know who, but somebody did. The question becomes whether the VC knew."

Professor Zelinsky said UON used a tool "to provide email security and protection", which holds external emails for review, and staff could either permit or block all emails from a sender, or allow one-off release.

"Last year, after some concerns raised from staff, we amended the email security ... tool to hold NTEU emails to enable staff the option to release, permit or block emails," he said.

"More than 85 per cent of our staff are not members of a union, but it is important staff have a choice as to whether they receive these emails."

Professor Zelinsky told the Newcastle Herald "previously [the emails] just passed through" and the move was intended to "give people a choice".

"Basically then there was a complaint made that the university was blocking emails and I said 'Look just don't filter it, let it go through until this vote is done'," he said.

"The union is trying to make out we've somehow done something untoward, there's nothing untoward here at all, it's all about being fair to everybody.

"I just didn't want - while we were coming into the vote - for people to think the university was doing something [wrong], because it could be spun one way or the other." He said he hadn't wanted the matter to be a "distraction".

The Herald reported on December 14 some staff who had permitted NTEU emails still hadn't received them.

"The university is not blocking or classifying NTEU emails as spam," Professor Zelinsky said at the time.

UON's information technology conditions of use policy says UON will not prevent the delivery of an email merely because it contains information relating to industrial matters.

NTEU NSW secretary Vince Caughley said any restricting of NTEU emails was "outrageous".

He asked Professor Zelinsky to make a public commitment that union emails would not be restricted in the future.

"This is a public university in receipt of hundreds of millions of dollars of public funding ostensibly for open research, open knowledge, fearless interrogation of fact, the public expectation isn't that university managements will block staff seeking alternative information or alternative views on things that are different to their management's," Mr Caughley said.

He said the NSW division lodged the freedom of information request after more than 600 people attended a staff forum around the time the proposed agreements were published, but "registrations dropped to single digits" for subsequent sessions.

He said staff also reported they weren't receiving the NTEU's emails.

Mr Caughley said the UON case had ramifications across the sector.

He said universities were "taking far more aggressive approaches to enterprise bargaining negotiations" and there had been anecdotal reports from members that other universities were "adopting a similar draconian approach to communications" and emails weren't reaching staff.

"All universities are on notice," he said. "Staff have a right to access information about their pay and conditions. Not everyone has to read or agree with NTEU communications, but that choice should be left up to staff."

Staff reviewed the agreements from December 6 to 12 and voted between December 13 to 15 to reject the agreements.