The efficiency of airfare search engines is shaky as their claims of best deals can actually be as much as double the price on a different site, the Consumer Council said.
The watchdog conducted a survey on six sites for flights that claim to help passengers find the best deal.
Skyscanner, Kayak, Cheapflights, Momondo, DuckDuckLook and Google Flights were tested between January and last month at least 50 times each using similar criteria.
This included searching for short-haul, medium-haul and long-haul flights.
The biggest variation was recorded in a return ticket from Hong Kong to Jeju, South Korea, for July.
The highest price - HK$4,102 on Google Flights - was nearly double the lowest price of HK$2,081 on Skyscanner.
The watchdog said the stark difference could be due to different airlines and departure times.
For the flight to Jeju, prices varied by over 20 percent from the cheapest - HK$2,081- to the most expensive at HK$2,536.
There was an even bigger difference of 45 percent for return tickets to Singapore in August as Skyscanner showed a price of HK$920, while DuckDuckLook listed the best deal as HK$1,337.
Quite often when consumers are diverted to the "cheapest" ticket, they could find it marked at a price as much as 30 percent higher, the council said.
In some cases, these tickets were sold out or not even listed on the websites.
The council found that in 17.3 percent of the time it used Kayak the lowest-priced tickets were sold out or the fares shortlisted could not be shown. Skyscanner and Google Flights also had similar problems.
The council found that among the six sites, the best performing was Skyscanner: for 35 percent of the time, it showed the cheapest airfare.
Google Flights fared the worst on nearly half of the occasions as the prices it displayed were higher than its competitors.
The watchdog reminded consumers to pay attention to "hacker fares" on those sites, which consist of different single flight tickets being combined into one return ticket.
"If you buy these hacker fares, you run the risk of violating the airline's policy of using the return ticket, and it is possible that you will not be allowed to board," the council's chief executive Gilly Wong Fung-han said.
"If you expect you can get on board with hacker fares, you may not be able to. So, this is a very big risk for consumers and you have to buy another ticket."
Clement Chan Kam-wing, chairman of the council's publicity and community relations committee, urged consumers to read the terms and conditions carefully.
"None of the comparison websites in the survey provided clear information in regards to baggage allowance, fare or booking class, air mileage earnings or ticket change and cancellation policies," Chan said.
The council's survey comes after Australia's consumer watchdog sued hotel price comparison website Trivago last year.
The company now faces a hefty fine of more than A$10 million (HK$55.36 million) for false advertising.