Police inquiries are ongoing and WorkSafe is also investigating the crash.
Senior Sergeant Al Fenwick said there was no evidence the brothers were doing anything wrong, so far.
Police want to hear from any witnessesand want to hear from a person in a red truck who stopped to help.
Meanwhile, Rotorua police are investigating a separate motorbike crash that killed a Ngongotahā man on Saturday.
Jarvin Ritai, 28, died in a crash with a power pole on Wikaraka St about 11.30am.
Rotorua Road Policing Manager Senior Sergeant Simon Sinclair said: "Any death in these circumstances is an absolute tragedy.
"As always we encourage all road users to take care out there, drive to the conditions and ensure you're not distracted or impaired."