The Commerce Commission is investigating the sale of Tournament Parking's car parks to Wilson Parking.
It says it will consider whether the acquisition is likely to have led to less competition in the market.
The competition watchdog doesn't give a reason for its investigation, and nobody from the commission has been available to comment.
However, it is inviting parties who consider they have relevant information to make submissions.
Established 14 years ago, New Zealand-owned Tournament Car Parking operates more than 80 locations throughout the country.
Wilson Parking, which has been operating for 40 years, says it's the country's largest private parking operator with more than 100 sites.
Its parent company, Wilson Group, is the largest car park operator in South-east Asia.