"My mom & cousin @linafanene were struck head on by a drunk driver this week - they lived. First reaction is to find the person who did this and do unrelenting harm to them," he wrote.
"But then you realize the most important thing is my family lived thru this and we can hug each other that much tighter these days.
"Hug your own family tighter today and be grateful you can tell them you love them."
He also used the hashtags: "#BearHugsAndGratitude #100PercentPreventable" and "#ChoicesMatter"
According to TMZ, Ata and Savelina were most likely in the white 2012 Cadillac Escalade the star gave to his mother as a gift.
Johnson has strong links to New Zealand, where he lived with his parents and attended Richmond Rd Primary School, in Auckland's Grey Lynn, for some time.
In a promotional video for the release of his latest film, Hercules, The Rock made a shout out to his family in Grey Lynn.
His mother, Ata Maivia Johnson, travels regularly to New Zealand. In 2008 she was in Auckland for the Samoan Sports Awards, in which her late father, wrestler Peter Fanene Maivia, was honoured and inducted into the Samoan Sports Association's hall of fame.
Savelina, who also goes by the name Lina and works for WWE's NXT division, posted a photo of The Rock visiting her in the hospital.
Without any makeup on and wearing prescription glasses, the beauty was almost unrecognisable. She pulled a funny face as she sat in her hospital bed.
Johnson has had a busy year. He not only promoted his big swords-and-sandals epic Hercules, but he recently completed shooting Fast & Furious 7 opposite Michelle Rodriguez and Vin Diesel.
He also shot San Andreas, about the aftermath of a massive earthquake in California. The Rock plays a rescue helicopter pilot in search of his daughter.
- Daily Mail