Australian conservationist Bindi Irwin has marked the 20th anniversary of her famous father's death with a touching tribute to the man best known as the "Crocodile Hunter".

Steve Irwin died on 4 September 2006, aged 44, after a stingray fatally pierced his chest while he was underwater filming a documentary at the Great Barrier Reef.

Bindi, who was eight when her father died, called Steve a "superhero" in an online post on Friday, saying she has "watched the world mourn my dad with our family" for the past two decades.

"I always thought that my dad's legacy is wildlife conservation; it turns out, his legacy is love. For our planet and for each other," Bindi, 28, wrote on Instagram.

She described the impact of her father's death on her mother Terri and her younger brother Robert, 22.

"My mum lost the love of her life; my brother and I lost the greatest dad," she wrote. "Grief never truly leaves us; we walk alongside it every day."

Bindi also shared details on how her daughter connects with the grandfather she never got to meet.

"I have a 5-year-old daughter who watches his documentaries every morning. She feels utterly loved by her 'grandpa crocodile' without ever having met him."

Since Steve's death, the Irwins have continued to run Australia Zoo in Queensland.

"My dad left a lasting impact so poignant that it will ripple out for generations to come," Bindi wrote, saying his life and legacy were "worth celebrating".

She thanked Steve for being the family's "guardian angel" and signed off her message saying she loved him "with my whole heart".

Steve's wife Terri also took to social media to mark the day, posting an interview where the khaki-clad wildlife advocate recounted Bindi's birth and its impact on him.

"The most incredible wildlife encounter of my life was the birth of my daughter, Bindi," Steve says in the interview, with his signature enthusiasm.

The first time he held his newborn child, the inspiration for her name hit him.

"I was flabbergasted and it was then that her name evolved. I've gone, 'Bindi!' after my favourite crocodile, Bindi," he said, with a wide grin.

Robert Irwin also posted a tribute to his father, with a photo of Steve holding him when he was a baby, alongside the caption, "Your legacy is forever".

Last November, Robert was crowned the winner of US dance competition Dancing with the Stars, a decade after his sister won the same title.