A diner was horrified to discover a dead mouse in his burger while enjoying an Australia Day lunch with his partner.

Bruce and Petra Blackburn were dining in the Civic Hotel in Perth, Western Australia, when the shocking incident occurred. They found the animal midday through their meal, they told local media.

"The third bite, I couldn't actually bite it. I realised there was a full-fledged hairy mouse in the meat patty," Mr Blackburn told 9News.

"My reaction was to just drop it straight away and feel sick."

His partner took a picture and posted it to her Facebook page. Since Saturday (January 26) it has received over 1,200 shares.

Their post read: Having AUSTRALIA DAY LUNCH and this is what BRUCE got in his Burger. Health dept issue.. he bit into a Full Mouse... F****** Yuk .......

Social media users were disgusted by the discovery:

Ashley Newman said: "Where's the vomit emoji?"

"Oh my god, that is so gross. Shut the kitchen down," said another.

But some incredulous users questioned whether their story was a hoax.

Lisa Boland said: "Really?? A burger is put together piece by piece. Pretty sure you'd notice a dead mouse."

Mr Blackburn said he received a full refund for the lunch but no apology from the restaurant. He planned to report the incident to the Western Australia Department of Health.

According to 9News, the hotel was investigating.