Hongkong Post said on Wednesday it has plugged loopholes in its computer system's programming code after a cyberattack on Sunday and Monday that saw its customers' personal information in the EC-Ship online portal being stolen.

Postmaster General Leonia Tai told an RTHK radio programme that the attack was launched through an EC-Ship account that had since been disabled.

She said information from 60,000 to 70,000 account holders' address book details had been stolen.

These include the names, addresses, phone and fax numbers as well as email addresses of both senders and recipients.

Hongkong Post has sent emails to all affected account holders to remind them to stay vigilant and ask them to pass along the message to contacts in their address books.

Tai said the loopholes were found in Hongkong Post's programming code.

"After we blocked the attack, we looked into how the incident had been able to happen," she said.

"We saw that our defences had worked but that they still needed to be upgraded to prevent the unauthorised party from accessing our system.

"In the midst of protecting the system, we found that the party was still able to access it after discovering some loopholes in our programming code, enabling it to retrieve the address book information of some accounts.

"So we immediately plugged the loophole."

She assured people that a similar incident will not happen again and added that the police force has launched an investigation.

She revealed that Hongkong Post is actively seeking advice from the Digital Policy Office on how it can step up security as well as also arranging "to use" the Central Internet Services, which comes with a multi-layered cyber security system that the office provides and manages round the clock.