Virgin Australia insists a dog which died on a Boxing Day flight to Sydney wasn't left out in the sun.

The French bulldog's owners believe he succumbed to heat stroke, with vets now warning of the dangers of flying with pets.

Bruno the French bulldog shared every moment with his owners, even flying to Townsville with them for Christmas.

But there was tragedy on the return flight with the six-year-old pooch found dead on arrival.

Owners Neil and Kristina Maccabee are accusing Virgin Australia of leaving him on the hot tarmac for 40 minutes.

Virgin Australia said it followed all protocols and procedures, adding the couple signed a waiver acknowledging the risk.

After reviewing security vision the airline said Bruno was in the shaded baggage area the entire time and had a drip foundation in his cage.

Despite this, it's launched an internal investigation.

“We are very sorry to hear of Bruno's passing as our team take great care and pride in ensuring guest’s beloved pets are safely transported around the country,” the carrier said.

According to veterinarian Lizzy Gan, flying with any pet can be dangerous.

Breeds like Bruno, with flat-nosed faces, are most vulnerable.

“They have difficulty dispelling heat, or expelling heat from their narrow airways,” Dr Gan said.