A spokesman for the bank said the non-customer facing employee contracted it from a close contact.
"As soon as they became aware a member of their household had contact with a Covid-19 case, the employee self-isolated.
"The self-isolation started last Friday evening (October 16) and official advice is that the staff member would not have become infectious until Tuesday of this week."
However, the spokesman said that as a precaution, the bank was deep cleaning the workspace and floor where the staff member worked.
He said other standard Covid-19 procedures, including asking the employee's colleagues to monitor their health, had been implemented.
"We're in frequent contact with this employee and are offering full support."
Nine new cases - eight in managed isolation and one household contact of one of the marine employee cluster cases - were reported in New Zealand today, director-general of health Dr Ashley Bloomfield said.
The Ministry of Health has been approached for clarification if the Westpac case is among those reported today.
There are 1923 confirmed cases and 66 active cases.
The one new community case is a close household contact of one of the cases from the marine employee cluster.
New Zealand's labs completed 6053 tests yesterday.
