Australian conglomerate Wesfarmers has said that the overhaul of the troubled Coles supermarket business would begin immediately, even though the appointment of a new chief executive is likely to be delayed for some months.

The company, which completed its A$20bn (US$18bn) acquisition of Coles last week, said it hoped to be able to announce a new head of food and liquor by Christmas.

"The supermarkets business is performing poorly and it requires significant change," Wesfarmers managing director Richard Goyder said. "We're not going to wait for the new CEO (of supermarkets). The change will be immediate, in coming days, weeks and months," he said.

Wesfarmers said it expects the overhaul of Coles to take five years, but improvements would be noticeable within the first year.

Wesfarmers' strategic advisor, Archie Norman, said he did not believe the gap with rival operator Woolworths, where food and liquor sales are currently growing at 7.6%, was insurmountable. "There is nothing they are doing that we can't match," he said.