Security guards must now stop anyone trying to enter a Work and Income office if they don't have ID.
The changes are part of a new government policy being rolled out after two staff members were shot and killed at the Ashburton office in 2014.
But the guards' union says it's causing long queues and frustrations at the door.
Mike Treen of Unite Union said many of the people arriving for their appointments did not know about the policy.
He said guards were working outside, on their own, with an appointment list and dealing with a queue of people.
"The guards aren't actually able to do their job of guarding the door because they're busy looking up and down checking the names.
"So they're not keeping their eye on the customers, not keeping an eye on what's happening outside. They're not able to do their job."
He said customers were being made to stand aside while contact was made with somebody inside the office - but the guards could only do that by tapping on the door.
Mr Treen said the system was confusing, frustrating and dangerous. "In effect you've got WINZ making the guards carry out reception duties."
In a statement, Ministry of Social Development associate national commissioner Te Rehia Papesch said it was an important part of security that they knew who was coming into their sites.
"Our security guards are having a conversation with people before they come into our offices to quickly discuss why a person is there, and helps to make sure we don't admit anyone who might represent a risk to the safety of other clients or our staff.
"People will be asked for ID - any form of ID. But we know not everyone carries this at all times so I can reassure people that not having ID won't mean people can't come into a Work and Income office."
Beneficiaries' advocate Kay Brereton said the ministry had told her 21 people had so far been turned away for not having ID, but she suspected the true number was far higher.