Queensland police say they have lost a valuable diamond ring swallowed by an alleged thief, because they failed to check his stools.

The 50-year-old man allegedly stole two diamond rings from an Indooroopilly jeweller last week and swallowed them.

He has been in police custody ever since, as officers waited for the rings to go through his system.

They now say one of the rings was inadvertently thrown out in a medical waste bag after the man passed it last weekend. An X-ray has confirmed the other ring is still inside the man.

Deputy Police Commissioner Ross Barnett says the officers on duty mistakenly believed the man would tell them when the ring came out.

“With the benefit of hindsight, it’s easy to sit here and say we should have conducted a secondary search regardless,” he said.

“It’s not embarrassing at all for me because the officers involved acted in good faith.

“This is a rare and difficult set of circumstances – no-one sets out to lose evidence.

“He obviously has an interest in retrieving and returning the rings – his bail is conditional on it.”

Police believe the ring was sent to landfill and say the cost of searching for it is greater than its value.

“We’re fairly confident that it’s probably under a huge amount of landfill at a refuse site and will not ever be recovered,” he said.

They are now discussing compensation with the ring’s owners and Police Ethical Standards Command is investigating the incident.

Deputy Commissioner Barnett says police will continue to allege the man stole two rings, using initial X-rays as evidence.

“I’m very confident that it will have no detrimental impact on the prosecution,” he said.

Second ring yet to be recovered

Police say they are now taking the man to Brisbane airport daily to use a special toilet used by the Australian Customs and Border Protection Service in similar cases.

The man has been under constant security camera surveillance and police have also been asking doctors what measures they can take to speed the passing of the second ring.

If it doesn’t come out, they may be forced to consider more medical intervention, like surgery.

“I’m sure, without having spoken to him, he probably regrets it,” Deputy Commissioner Barnett said.

“It’s not a pleasant way to spend a week of your life and it’s not over yet.”

Deputy Commissioner Barnett says police have dealt with other cases of missing evidence before, but never one like this.

“This is rather unique, in terms of the circumstances in which we find ourselves,” he said.