The crowd erupted in jubilation after the girl accepted the proposal.
Rob popped up the question during the morning session on Day 3 in front of a packed crowd at the Gabba, Brisbane.
In a video, which is doing rounds on social media, Nat was left surprised by the proposal but said 'yes' moments later. As soon as she accepted the proposal, fans around them erupted into cheers inside the stadium.
"I'm going to keep this short and sweet. It's been four years. Will you marry me?", Rob said while pulling the ring out and getting down on a knee.
The couple was later interviewed by 7 Cricket and Nat admitted that she was taken aback by the proposal.
"It wasn't expected at all. I'm still so shocked," she said.
Recalling their first interaction, Rob said: "I met her at the cricket (game). We met at the Melbourne Cricket Ground (MCG) four years ago."
Coming to the match, the hosts are in a commanding position having taken a massive first-innings lead of 278 runs.
After bowling out England for a paltry 147 in the first essay, Australia piled up a massive 425/10 on the back of Travis Head's maiden Ashes ton (152).
Batting in the second innings, the visitors posted 220/2 at stumps on Day 3, still trailing Australia by 58 runs. Skipper Joe Root (86*) and Dawid Malan (80*) are unbeaten in the middle.