The Waitemata Serious Crash Unit is investigating, however witnesses have told police that it appeared the motorcyclist was speeding just before the crash.
"It's possible that speed is factor. There were only two vehicles involved, him and the truck. We are still investigating but according to people at the scene speed was possibly a factor on his part."
A St John spokeswoman said the motorcyclist was transported in a critical, or status one, condition.
She said the truck driver was assessed to be in a moderate, or status three, condition and was taken to Waitakere Hospital.
Royal Rd was still cordoned off at 8pm, which caused traffic to back up on the motorway.