The picture shows one half of the sandwich with a tiny, fatty bit of beef and some garnish.
The butter doesn't even appear to go to the edges as it should do in all good sandwiches.
"Delicious roast beef sandwich enjoyed by a Qantas frequent flyer today. Neil Perry, take your hat off. #qantas @QantasNews," Mr Purvinas wrote.
The sandwich has no affiliation with Neil Perry, consulting chef for Qantas.
Attached to the photo was another caption which said the passenger expected more, even though they did not have high expectations what they would be served on the flight.
The post prompted a flood of comments from people who could not believe what had been served up.
One man said he had never understood airline economics.
"Granted, these are all very first world problems, but why would a 'premium' airline such as Qantas skimp on something so financially inconsequential as the in-flight F&B service, when it often drives flyer loyalty & word-of-mouth?," he wrote.
Mr Purvinas suggested it was a sign the airline's cutbacks had gone too far.
"They are short of pilots, engineers, flight attendants, baggage handlers and sandwich makers. More managers than ever though," he wrote.
"This half arsed sandwich appears to be quite common. I'm following the pages and comments of frequent flyers and many of them agree.
"My comments being made in the hope that the airline rectifies the issues. My direct approaches have fallen on deaf ears."
In a description on his Twitter profile, Mr Purvinas says he was an aircraft engineer for 21 years for Trans Australia Airlines, which later became Qantas.
Other users said they would be calling for loads of frequent flyer points as compensation and that even Jetstar served up better food than that.
"It's a shocker and always is," said another.
"I travel frequently from Sg to Bne and the meal never changes and is very poor."
A Qantas spokesman said: "We can completely understand that the customer would have been disappointed with their meal.
"We'll be following up with our supplier and making it very clear that providing a filling sandwich is their bread and butter."
Homemade Food Co told news.com.au they had dealt with Qantas and the individual directly and did not want to make further comment to the media.
