More than 100,000 Aucklanders have an AT Hop card that is not registered with their name and contact details for Covid-19 tracing purposes.
Auckland Transport is actively encouraging people to register their Hop cards after two people tested positive for Covid after taking a number of buses while infectious.
It comes as the Government today decided to make masks mandatory on public transport from Monday and one of today's eight cases of Covid in the community had contact with a confirmed case on a bus.
The Prime Minister said face masks would now be mandatory on all public transport across New Zealand at level 2 and above.
The push for more Hop cards to be registered also follows a revelation that two bus passengers who were close contacts with the two infected people were travelling on an unregistered Hop card and authorities haven't found contact information for them.
An AT spokesman said there were about 1 million active Hop cards, of which about 700,000 were in regular use.
Of the regular users, 83 per cent of the regular users were registered, he said.
This means the other 17 per cent, or about 120,000 regular users, are not registered.
The AT spokesman said mobile alerts and other communications were being used to encourage people to register their Hop cards for tracing purposes.
Prime Minister Jacinda Ardern and director general of health Dr Ashley Bloomfield encouraged the registration of cards at two of their media press conferences last week. The Auckland Regional Public Health Service did the same at the weekend, he said.
On social media, AT has a video post of Facebook and twitter to drive registration of HOP cards. Customer service centres have been encouraging registration and call centre staff have been phoning customers to get them to register their cards.
"We know that some people will not register their card for various reasons and legally that is their right."
The spokesman said the Ministry of Health, through the Auckland Regional Public Health Services, did not have direct access to Hop card data, but if AT was required to share information under the Health Act, it was transmitted by secure file transfer protocol, not by email.
That protocol was used for the first time on Friday.
"If a request is made, we only share customer details deemed critical to assist with contact tracing such as name, address, phone number and travel details.
"AT will never share credit card details or bank account numbers. In fact, we don't hold or store any payment information," he said.
The spokesman said AT was continuing with spot cleaning and disinfecting high-touch areas on buses and trains up to twice a day, and completed another round of fog sanitation, which occurred during the first lockdown.