Capt. Francois Moller says he obtained more than 1,000 exchanges between Pistorius and Reeva Steenkamp.
Moller said he received as evidence two BlackBerry phones, two iPhones, two iPads and a Mac computer from Pistorius' house the day after Steenkamp was shot to death.
He began to discuss Tuesday how he pulled data from the phones.
He said the data on Steenkamp's phone would print to more than 35,000 pages.
It gives insight into the couple's relationship before Feb. 14, 2013. Prosecutors allege Pistorius fatally shot Steenkamp after a Valentine's Day argument. Pistorius says he killed her by accident.