"For 20 years you have brought endless amounts of light into this world," Irwin said of her boyfriend in a sweet Instagram post on Sunday. "I am thankful every day to have you in my life. We've traveled the world together and had some of the most extraordinary adventures. I'm so excited to continue on this beautiful journey with you."
The former Dancing With the Stars winner's heartfelt message was accompanied by a photo of herself holding a koala while posing with Powell. "Your kindness and strength is inspiring, and your smile will always be my favorite," she continued. "Happy birthday my sunshine, you're amazing."
Powell also shared how grateful he was to be celebrating his birthday with Irwin. "Twenty years old will definitely be the most difficult age for me to get used to thus far," he captioned a photo of himself and his 18-year-old girlfriend. "I couldn't have asked for a better way to spend it than living in the middle of nowhere for a few days with my girl @bindisueirwin. She sure knows how to give me an amazing birthday."
In another Instagram post, Powell shared a video of the couple enjoying a moonlit night together. "Thank you for the supermoon for my birthday, Australia! Couldn't have wished for a more amazing day with @bindisueirwin," he wrote. "Spent the day hiking the Great Dividing Range, and then came back to watch the moon rise on the roof with my girl. Looking forward to #SteveIrwinDay tomorrow @australiazoo!! So happy."
These two don't just gush over one another on Instagram. In their first joint interview last month, Irwin shared with ET the "defining moment" when she first realized Powell was the one for her. Check it out: