The 30-year-old star confirmed the exciting news via her Instagram account.
Sharing a touching photo of her new family-of-four, including her two-year-old son Bodhi Rain, Teresa penned: "Our sweet littlest love is here. Forest Sage Palmer was born yesterday in Adelaide on 12/12 at 12:18pm weighing 8lb 4oz and 20 inches long!"
"Our hearts are so full and blessed, he is perfect! Picture by our girl @gemma_peanut."
Throughout her pregnancy, Teresa has kept fans up to date with her progress through her searingly honest blog called Your Zen Mama.
In a recent interview on The Kyle and Jackie O Show, the Hacksaw Ridge* star said her second pregnancy hadn't been too bad and she wanted at least six kids.
"Mine's been really cruisey. I'm super lucky, touch wood. Hopefully everything goes well with the birth," she told the radio hosts.
"It's been really good. I've definitely packed it on second time round!"
"Six is my number! It truly is. My husband is down. He's like, "Why not?'" She added of her expanding brood.
Congrats to Teresa and Mark on their new bundle of joy!