UPDATE 5.30pm: The speed limit on the Western Highway remains at 80kmh, while Victoria Police say there are no updates from Major Collision Investigation Unit detectives.
A spokeswoman said the investigation would likely take some time, as detectives took statements from all the bus passengers and witnesses to Thursday morning's fatal incident.
The cause of the truck and bus crash near Pimpinio remains unknown.
UPDATE 10.15am: Major Collision Investigation Unit detectives are continuing to investigate the circumstances surrounding Thursday's fatal crash at Pimpinio.
A Victoria Police spokesman said the female passenger of the truck - a Murray Bridge woman, 22 - was arrested on Thursday, allegedly in possession of drugs.
Police have charged the woman with traffick a drug of dependence, possess a drug of dependence and use a drug of dependence.
She has been remanded in custody to appear at the Ararat Magistrates' Court on Friday.
The spokesman called on anyone who witnessed the incident, or with dash cam footage, to contact Crime Stoppers on 1800 333 000 or submit a confidential report online at www.crimestoppersvic.com.au
EARLIER: The Western Highway has re-opened to traffic in both directions between Horsham and Dimboola following a fatal crash on Thursday morning.
Emergency services were called to the scene at Pimpinio about 2am after a passenger bus crashed into a B-double trailer, which had dislodged from the cabin.
Both vehicles were travelling towards Horsham.
The bus driver, a Ferntree Gully man, 60, died at the scene.
Wimmera Base Hospital treated more than 40 people who were travelling on the bus at the time. Four people were flown to Melbourne.
The truck driver, a Murray Bridge man, 36, and his female passenger, 22, were not injured.
The truck driver was released from police custody on Thursday night night and is assisting police with their inquiries.
Investigations into the cause of the crash are still ongoing.
The woman remains in custody this morning for other matters.
The highway reopened about 7am. An 80kmh speed limit is in place at the stretch of highway where the crash occurred.
A VicRoads spokesperson said people could access the latest traffic information around the state via traffic.vicroads.vic.gov.au
While you're with us, you can now receive updates straight to your inbox twice weekly from the Wimmera Mail-Times. To make sure you're up-to-date with all the news from across the Wimmera, sign up below.