"We have no idea at this stage, inquiries are obviously continuing. It's a bit of a blank canvas... until we talk to all the witnesses we don't know."
Police would speak to several "witnesses" who had passed the scene of the crash, and others who had called in reporting hearing motorbikes in the area, Mr Metcalfe said.
"Obviously at the moment our priority is at the scene and getting that cleared so we can open the roads.
"Then there will be quite extensive inquiries into what happened on the road, and what actually lead up to him being on the road."
Police were yet to identify the man, Mr Metcalfe said.
SH1 north of Apotu Rd was closed and diversions were in place via Saleyards Rd.
The road was expected to be reopened sometime this morning.
The serious crash unit remains at the scene.