Cartels involve illegal agreements between competitors not to compete with each other, such as price fixing, the restriction of outputs, the allocation of customers, suppliers or territories, and bid rigging.
They are hard to detect and harm the economy by removing the benefits of competition, leading to higher prices and less choice for customers, according to the commission.
Regulators in Hong Kong and New Zealand said this month that they were investigating banks' conduct in the foreign exchange market as part of similar investigations around the world, including Singapore, Britain and Switzerland.
About US$5 trillion is traded each day on foreign exchange markets and the Kiwi dollar is one of the world's most actively traded currencies.
Market authorities worldwide are looking closely at traders' behaviour on several key benchmarks, spanning interest rates, foreign exchange and commodities markets.
Several banks and brokers have already been fined billions of dollars for manipulating benchmark interest rates such as the London Interbank Offered Rate (Libor).
Authorities are looking at whether traders from different banks colluded to influence currency prices for their own benefit at the expense of clients.
Massey University senior banking lecturer David Tripe said the probe seemed linked to others abroad. "But I would be reluctant to conclude that there was widespread collusion among the local banks based on today's announcement."