Emergency services were called out about 7.45pm, by which time the parents of the girls had managed to free one.
But by then the ordeal had been dragging on for several hours.
Hokitika fire chief Harry Collett said the teenagers, who lived locally, had gone for a walk in the riverbed above the Kaniere Bridge on the Woodstock side yesterday afternoon.
They got bogged down and stuck in "solidified sand" in a backwash of the river.
"They were stuck for six hours. The parents turned up and managed to get one out after three hours. It was bizarre," Mr Collett said.
"They were well and truly stuck."
Mr Collett said there was no cellphone coverage at the point where the pair got bogged, and their ordeal only became known when a local resident heard their yells for help after two to three hours.
"They weren't hurt - they were just stuck, waist deep."
Mr Collett said it was not clear why emergency services were not immediately called when the parents were notified and began to free them.
When the brigade arrived the task did look deceptively straightforward, he said.
"It looked so simple but it was so bizarre and it took a long time ... you'd think 'this can't be hard."
When the brigade arrived portable lighting was set up and fire volunteers worked digging by hand for about 90 minutes until they freed the second girl, about 9pm.
"We had to keep digging and digging and digging... It was hard work. It was pretty physically demanding "
Mr Collett said it counted as one of the more unusual fire calls.
Some of his volunteers had commented that it was a more physically daunting task than a house fire.
The girls, who are known to live locally, were otherwise physically unharmed following the ordeal.
