An All Blacks' spokesperson told reporters ahead of the team's final training session at Lancaster Park that Mathewson had left the squad to fly to Wellington where his partner Cara had gone into labour.
"He'll be flying back first thing in the morning (on Saturday)," added the spokesperson.
Coincidentally Mathewson was drafted into the All Blacks' squad ahead of last week's match in Melbourne to cover for Piri Weepu, who had stayed behind in New Zealand while his partner gave birth to their first child.
Weepu rejoined the side in Melbourne after the birth of his daughter and came off the bench to replace Jimmy Cowan, who suffered a rib injury in the 49-28 victory and was ruled out of this Saturday's clash.
Weepu will start the match with the uncapped Mathewson, who had an outstanding Super 14 season for the Blues and was considered unlucky not to be named in the original All Blacks squad earlier this year, on the bench.