The shop workers' union is sceptical of a survey suggesting two-thirds of restaurants in South Australia closed last Christmas Eve and New Year's Eve, rather than face higher penalty rates.

The survey by Restaurant and Catering SA estimates the closures cost the industry $11 million and the state lost $1 million in GST revenue.

"[The] survey makes it clear that the impact on the community is a significant one. The Government has made a decision that has the potential to make South Australia a ghost town over the Christmas holiday period," it said in a statement.

The survey said more than 66 per cent of venues had closed compared with 19 per cent the previous year.

Part-day public holidays were introduced last year as part of the deal for longer shopping hours in the city.

Shop union secretary Peter Malinauskas said the survey figures needed closer scrutiny.

"They seem to be completely inconsistent with any anecdotal evidence," he said.

The Centre for Economic Studies is reviewing the effects of the retail changes.