Chief Executive John Lee said on Tuesday that the Covid infection risk on the mainland is the lowest in the world and this is why a rule requiring people arriving in Hong Kong to be vaccinated doesn't apply to mainlanders.

Lee was responding to an online post purportedly by a mainland visitor who said they had obtained a temporary vaccine pass at a post office in the SAR, even though they were not vaccinated and had no exemption certificate.

"For mainlanders not having to get vaccinated to come to Hong Kong, that has been a policy for a long time. The main reason is that the Covid risk on the mainland is the lowest in the whole world. Everybody knows the standard to control the pandemic there... The risk [on the mainland] is much lower compared to Hong Kong," he told reporters at a weekly media session.

"Of course we will keep monitoring the overall Covid situation... but so far we do not see any imported cases as a result [of this policy]."

A government statement in May said inbound travellers from the mainland or Macau could get a temporary vaccine pass valid for six months.

Meanwhile, Lee said SAR officials are still in talks with Shenzhen authorities on allowing people travelling to the mainland to do their quarantine on this side of the border.

He said there are "a lot of challenges to overcome" but believes an agreement will be reached.

"What I intend to [do] ... is to create a system which in the long run, however the situation of the pandemic develops on either side of the boundary, can continue to ensure that it will proceed without big changes," he said.