South Africa's Postbank has been forced to replace 12 million bank cards after a calamitous security breach that saw the bank's master encryption key printed off in plain, unencrypted language.

According to internal documents acquired by the Sunday Times of South Africa, the 36-digit code security key “allows anyone who has it to gain unfettered access to the bank’s systems, and allows them to read and rewrite account balances, and change information and data on any of the bank’s 12-million cards".

The master key was apparently printed out on plain paper in a data centre in Pretoria in 2018, enabling the fraudsters to make over 25,000 fraudulent transactions, mostly from cards used by people receiving social benefits from the government.

The crime, which is being pinned on a number of rogue bank employees, went unnoticed for months. More than $3.2 million was stolen in the raid.

The cost to the bank of replacing all the compromised cards is expected to reach $58 million.