THE Australian retail sector will continue to restructure amid a gradual pickup in sales in the next 12 months as its economy emerges from recession, Standard & Poor's Australian arm says. S & P predicted in a profile of the sector that retail sales would grow about 2.5 per cent in the year ending May 1993. Coles Myer, which operates department stores, discount stores and grocery and specialty chains, would maintain its dominant position in the Australian market, particularly with the breakup of the Adelaide Steamship group's retail businesses, the ratings company said. Restructuring of the wholesale grocery sector, which has been a feature of the sector in the past 18 months, was likely to continue, S & P said. ''Further restructuring is expected to continue over 1993-94, which will most likely result in a greater concentration of market share to the larger companies,'' S & P said.