The power of a combined Lyttelton Port and Port of Otago is worrying one of the world's largest shipping companies, Maersk, the Taranaki Daily News reported.

Lyttelton Port Company (LPC) has been given the green light from its main shareholder, Christchurch City Holdings (CCHL), to merge with its main South Island rival, Port Otago.

The ports will merge operations, but not their assets, and not for some months because the merger plan must first pass Commerce Commission scrutiny.