He spoke with Anh Do in a candid interview on Anh's Brush With Fame about watching his father come home with his sister Connie, and curl up in a ball, sobbing.
"I’ll never forget the moment, the afternoon that I found out," Johnson said.
"(Dad) drove an old s***ty blue Torana at the time. I looked through the window, and my sister was in the front seat (huddled), like this, wouldn’t look anywhere.
"Then I looked over at my dad, and dad was just white. I was like, 's**t" something’s gone wrong here man'.
"He walked inside, and ignored me. I followed him inside, and he was curled up on the couch, in the fetal position, sobbing just weeping, just bawling."
While Connie was first diagnosed with cancer when she was 11, she has struggled with the disease her whole life. A tumour was found in her womb at 22, and has battled breast cancer, and last year she got news of more cancer growing in her liver.
Now 40, she has two children, and started a charity called Love Your Sister with Samuel in 2012 to raise money for the Garvan Institute of Medical Research. The charity aims to fundraise to find a cure for breast cancer, and Samuel has ridden a unicycle around Australia for the cause.
Seven years ago, Connie was given six months to live. She has since undergone a double mastectomy, radiation, chemotherapy and more medical procedures to fight the cancer.
Sadly, earlier this year she was told that medically, she can't have more chemotherapy. She told news.com.au the news, "I guess my organs are just saying, ‘No more. No more.”