The competition regulator has expressed concerns about the sale of four WA Progressive Supa IGA stores to Coles.
Metcash and the co-owners of the stores in Busselton, Halls Head, Bunbury and Dianella plan to sell the supermarkets to the Wesfarmers-owned retail giant.
The Australian Competition and Consumer Commission (ACCC) said its preliminary view was that the proposed acquisition might result in a substantial lessening of competition in one or more of the local markets.
There are no other Supa IGA stores in at least three of the four local markets, the regulator noted.
"Market participants have expressed concern about losing the differentiated offering of their local Progressive Supa IGA stores," ACCC commissioner Dr Jill Walker said in a statement.
"The ACCC considers that the differentiated offer of the Supa IGA stores reflects a competitive response to rival supermarkets and provides additional choice to consumers.
"The ACCC is also concerned that the proposed acquisition would remove access to Supa IGA promotions for shoppers in these areas."
The regulator said it welcomed further submissions from the public by July 24 on email firstname.lastname@example.org .
The final decision has been deferred until August 14.