Coach Darren Lehmann told the 19-year-old that he would be playing in the opening Ashes Test two days before the match got underway.
"The coach, Darren Lehmann, told him on Monday to ring his folks back home, so we just dropped everything and flew over," The Sun quotes his father, John Agar, as saying.
"He may have fallen two short of a century but we don't care. We are over the moon. He's actually a very good batsman, you'd like to think one day he'd move higher in the order!"
His mother, Sonia, added: "When he was on nought, I was most nervous, I only started to relax when he got to 20."
Agar made 98 runs with the bat in his first innings as a Test player, which broke the world record for the highest score by a number 11 batsman.