They tied the knot in surprise shortly after strict lockdown restrictions were announced.
Powell commented on his wife's latest post, "Our little Wildlife Warrior is going to be so loved".
Fans flooded the post with excited comments.
"I don't think I've ever been more excited/emotional for a pregnancy announcement. I'm so excited for you guys," wrote one woman.
Last month Irwin revealed she was in her first trimester, and asked fans for "advice, good vibes and prayers".
"Baby Wildlife Warrior due 2021," she wrote.
"Chandler and I are proud to announce that we're expecting! It's an honour to share this special moment in our lives with you. Though I'm still in my first trimester, we really want you to be part of our journey from the beginning of this new life chapter."
Gushing over his wife, Chandler took to the comments to write: "You're going to be the best mother."
In a sweet post a month on from her wedding day, Irwin shared the vows she made to her 23-year-old husband.
"Chandler, when I went to write my vows I found I couldn't stop writing them," she began.
"I started by sharing all the ways you make my life beautiful and wrote close to a thousand words. Then I moved on to write about what an extraordinary person you are and I needed a new pen. Finally I wanted to share all my hopes for the future and I ran out of notebook paper."
Irwin said what she and Powell shared was "genuine, unconditional" love that was "meant to be felt in every part of our soul".
She got engaged to Powell last July after he proposed on her 21st birthday at her favourite location in Australia Zoo.
