"He was with us from the beginning of the tour and worked really hard on his skills," said South Africa assistant coach Vincent Barnes.
"It was fantastic to see the improvement in him in Perth.
"It was a wonderful moment for him. Both players are prepared to listen and put in the hard yards."
Barnes continued: "When Parnell joined up with us in Australia (for the one-day series) he came to me and said he would like to be a member of the one-day group for a long time and asked my advice on what he should work on.
"It’s remarkable how quickly he slotted in as a 19-year-old. I believe he will play for the Proteas for many years to come."