They got a phone call six minutes later telling them their North Linwood home had been broken into.
"The first thing I did is look at my dresser and my jewellery was gone. I saw the open box and thought, and so is my son," she said.
Around $20,000 worth of jewellery, their passports and photos were also missing.
A police spokeswoman confirmed police were investigating the burglary, which was reported on Thursday.
"I lost my child a long time ago - and now I've just lost him again," she said.
The Wahnigs' lost their son Thomas at birth.
He would have celebrated his 21st birthday in September last year.
"We didn't want to bury him in the cemetery because he was too little. The first one of us that went was going to take him with us, but now that's gone.
"I keep looking, thinking maybe they've dropped him back. But, obviously these people have no conscience. They have no heart."
Mrs Wahnig suspects the burglars thought her son's ashes were drugs.
"His ashes were a creamy colour, not white," she said. "Thomas doesn't deserve to be thrown away once the scum realise that he isn't what they first thought it was."
Jewellery from her late mother were among the pieces taken, something she says is irreplaceable.
She asks if anyone saw anything suspicious along Buckleys Rd, North Linwood, in Christchurch, around 4pm Thursday to contact police.
