Cheuk Shun, who chairs the Union of Hong Kong Post Office Employees, said staff may have to work overtime to process the queues.
The government said on Thursday that it was increasing staffing at post offices across Hong Kong to handle the queues. Overall daily quotas have been doubled to allow 2,400 people to sign up each day, and that will be increased to 3,000 next week.
An online registration service will soon reopen, and details will be announced later.
“Do we need extra working hours to handle the applications? Now it takes around five to 10 minutes for each application, and some of the applicants may not have all the information needed with them, so that takes longer,” Cheuk said on an RTHK programme.
For example, he said some information may be missing for those who carry mainland vaccination QR codes, but those with mainland paper records are generally fine.
Cheuk added that general postal services may also be affected by the bigger crowds.
Queues were seen for a second straight day at a number of post offices.
In Wan Chai, a woman was among those who started queuing hours before the post office was due to open.
“I came the day before but they ran out of tickets, so I queue again. I arrived at around 6:40am for my daughter, she got vaccinated in Canada,” she told reporters.
She said her daughter could carry her paper jab record around for the vaccine pass but found it to be inconvenient.
A man from the mainland said he’s in Hong Kong for an exam, and he just learned that he needs the vaccine pass to enter the venue.
“It’s a bit inconvenient, I didn’t know I couldn’t enter the venue without the pass. Queuing may take up some of my energy, but it’s okay, I just come over a little early, from around 7 in the morning,” he said.
Others in the queue said for months they had been able to enter venues by providing overseas vaccine records for scanning, but that changed recently.