Long queues formed at Shenzhen Bay Port on Monday as people heading to the mainland waited for Covid tests and then their results.
Some complained that they had to wait outside as the indoor, air-conditioned areas were already full.
A man surnamed Chan was among those who arrived in the morning and waited in a queue outdoors.
"[Travellers] should be arranged to come here in batches, when they get a hotel room [on the mainland]," he told RTHK.
Another traveller, surnamed Ho, said the authorities should deploy more manpower to help with Covid testing.
On Sunday, the government advised people not to head to the border checkpoint in the morning, saying waiting times would be reduced if they travelled there later in the day.
The government also said at the weekend that it is planning to introduce an online booking system for people to arrange their Covid tests at the border.