Andy Reeve, son Shane and grandson Levi, of Dunedin, all celebrate their birthday on March 30.
The chances of three males from three generations of one family being born on the same day is one in 194 million.
Shane said he knew the situation was unusual when he and his father celebrated together, but having Levi born on the same day made it more special.
‘‘Because I was born on Dad's birthday ... it's always been like that. Then he [Levi] came along and it was just another one to add to the day.
‘‘It's a day that's going to be very special for us for a while,'' Shane said.
When Levi was born on the same date, Shane said he was shocked by the coincidence.
‘‘I didn't expect it.‘‘
I rang Dad and said ‘come meet your early birthday present'.
‘‘I think he's feeling quite left out [now].''
The coincidence was equally unexpected for Andy. He had never heard of it occurring before.
He said having wife Denise give birth to a son on his birthday was ‘‘special'' and when Levi was born it became even more so.
The family would celebrate with a ‘‘big party'' in the coming weeks but would spend today with extended family.
King's High School physics teacher Jesse Farry, who is also a trained mathematics teacher, said three male generations of one family being born on the same day had odds of one in 194,508,500.
The chances of three generations from the same family of any sex being born on the same day was one in 48,627,125.
Mr Farry took into account that Andy did not choose March 30 as his birthday so calculated the chances as one in 365 days per person, or 365 cubed.
If Andy is not counted, the chances of three generations of the same family having a birthday on the same day is one in 133,225.
This was more appropriate, Mr Farry said.
He said he enjoyed working out probability and chance ‘‘when they come along''.
