A MAROOCHY River mother does not know how to tell her children the family dog will not be coming home after the lost pet was put down following an apparent information mix-up.

Kylie Limpus's pet dog Max, a 10-year-old female cattle dog-boxer cross, was put to sleep on Friday at the RSPCA-operated Noosa pound, a day after Ms Limpus had left her details with the Tanawha pound.

Ms Limpus believes a mistake which saw her dog listed as a border collie at Noosa meant Tanawha staff were unable to properly search for Max.

Sunshine Coast Council has begun an investigation into what went wrong.

"I'm angry now because we could have saved her," Ms Limpus said. "Max was at Noosa when I asked at Tanawha if she was in the pound. No one ever suggested I try Noosa.

"If they're going to send dogs to Noosa they need to tell people they could be there."

Ms Limpus realised Max was missing on Wednesday, August 28.

At that point it had been a few days since she and her neighbours, who sometimes look after Max, had seen her.

The council's lost pets web page showed Max was found in the Maroochy River area and impounded on Thursday, August 22.

Ms Limpus was directed to the same page on Saturday, when she discovered Max was at Noosa.

Ms Limpus went to the pound on Sunday thinking she would be bringing Max home. Instead she was told the family dog had been put down.

Max was not micro-chipped but was registered, though she was not wearing tags when she went missing.