In a heartfelt post on Instagram on Saturday, Robbie said he was celebrating the birthday of his twin brothers, Pua and Tanumapua Magasiva.
"Today I will celebrate the twins birthday. One I miss every day, the other I’ve got your back every day. Happy birthday Tanumapua. #missyoupoos #Ivegotyourbacktanu," the post said.
It was accompanied by pictures of the brothers as adults and as toddlers.
Pua, well-known for his role on Shortland Street, Sione's Wedding and Power Rangers, was found dead in unsuspicious circumstances in Wellington in May.
Many other Kiwi stars replied to Robbie's post, sharing their love.
"Such a poignant and difficult day, sending you & your family love today," wrote Jaquie Brown, while Ma'a Nonu and Shortland Street actress Ria Vandervis commented heart emojis.
"Manuia Lou aso Fanau usos," wrote actor Beulah Koale, translating to Happy Birthday.
Previously, Robbie got a tattoo in honour of his late brother.
Photo Robbie Magasiva Instagram