In the age of social media, restaurants are learning that only a twit ignores an unhappy customer.
Almost a quarter of bad dining experiences are posted on social media within minutes of leaving a restaurant, and some disgruntled diners even vent their anger online while they are still seated.
Restaurant and Catering chief executive John Hart said restaurants could ill afford to ignore social media.
"The prevalence of smart phones and increasing ease with which diners can congratulate or condemn their dining experience means the power of social media is the sleeping giant that cannot be ignored," he said.
Mr Hart said restaurants needed to resolve problems with a customer immediately, or be doomed.
"Waiting to rectify a problem with patrons at the end of the meal may be too late," he said.
Generation Y diners are the most likely to express their dissatisfaction online, according to the Galaxy poll of 600 people for American Express.
Forty per cent of Gen Y said they shared dining experiences on Twitter or Facebook, compared with 26 per cent of Generation X and 19 per cent of baby boomers.
The good news for restaurants is that 85 per cent of good dining experiences are put online within a week, and more than 62 per cent are posted within 24 hours.However, 87 per cent of bad dining experiences are shared online within a week, and three-quarters are uploaded within a day.