Five others were taken to hospital after the crash during the Round the Mountain charity toy run.
A fellow charity rider Lyall Broadmore, who was ahead of the bikers involved in the incident, said he passed a van full of people up to "stupid antics" shortly before the crash.
One of the passengers, a man, was sitting with his upper body leaning out of the window of the van as he waved to the motorcyclists.
"They were encouraging the bikes, waving to the bikes ... I thought it was an accident waiting to happen."
Mr Broadmore said he could not be certain if the same van was involved in the crash moments afterwards, but it was not far from the scene of the accident.
He said the group of motorcyclists decided to continue with the charity run after the crash because they were "there for the kids who go without".
A 24-year-old Wanganui man, thought to have been driving the van, remains in a critical condition in Waikato Hospital, where he was flown from Taranaki Base Hospital.
Three other men injured in the crash remain in Taranaki Base Hospital.
A hospital spokeswoman said two, aged 22 and 27, were in a comfortable condition, and a 62-year-old was described as stable.
Meanwhile, a 42-year-old man, who was also injured in the collision, had been transferred to Middlemore Hospital in Auckland in a serious but stable condition.
The weekend fatalities have brought the road toll for the year to 274 - up from 255 at the same time last year.
National road policing manager Superintendent Carey Griffiths said the key safety message for motorcyclists was, if they were travelling in groups, to keep distances between each other.
"So if something untoward happens it doesn't take a whole lot of bikes down at the same time."
He said the same messages still needed to be emphasised to all motorists; keep to the speed limits, pay attention, wear seatbelts and don't drink and drive.
On Saturday, two other motorcyclists and two people in cars were killed in separate accidents and 10 others were injured in crashes.
Among those injured were three males, aged between 16 and 22 in a crash on State Highway 2 north of Wairoa.