Mr Thompson, 31, was on his way to pick Koen up from preschool to meet his new sibling, and see mum Shannon Weir, 27. But the "heartless" thieves meant he was stranded.
"Who would do that?" a bewildered and still upset Mr Thompson said yesterday.
"It really annoys me. We moved back from Australia to help rebuild Shannon's home town, and things like this happen ... it's such a shame."
Ms Weir went into labour about 8pm on Thursday night.
A midwife suggested they hurry from their home in Hornby, west Christchurch, into hospital.
There were soon complications - the umbilical cord wrapping around unborn Carter's neck.
"Things took a real turn for the worse and I thought I was going to lose them both," Mr Thompson said.
"About 15 doctors came in and they decided to do an immediate c-section. Thank God they're both okay."
The pre-cast concrete worker praised the quick and professional actions of the medical team.
And he was impressed with the police officers now trying to track down his maroon Nissan Bluebird.