The Australian Competition and Consumer Commission (ACCC) is reviewing retailer Woolworths' proposed 55% stake buy in PETstock.

The concerns relate to PETstock's acquisitions of Best Friends Pets, Pet City, Animal Tuckerbox and Pet and Aquarium Warehouse in Eltham, Victoria between 2017 and 2022 that were not notified to the ACCC, the watchdog said.

PETstock did not immediately respond to Reuters' request for comment.

"During the current Woolworths-Petstock merger review, market participants expressed concerns about the already significant consolidation that had occurred within specialty pet retail in recent years," ACCC Commissioner Stephen Ridgeway said.

PETstock's acquisitions are all pre-date to its deal to acquire the 55% stake, Woolworths in a separate statement said, adding that the company will engage with the ACCC as part of the consultation.

PETstock has offered to divest multiple sites and assets it acquired in these completed acquisitions, ACCC added.

(Reporting by Ayushman Ojha; Editing by Sriraj Kalluvila and Sherry Jacob-Phillips)