Dozens of emergency service workers have been dealing with the chaotic scene on the Sunraysia Highway, around 10 kilometres northwest of Avoca in the state's west.
The woman died at the scene.
Six passengers were critically injured — three of them flown to Melbourne and three taken to Ballarat Hospital. One of those flown to Melbourne has been downgraded to a serious condition.
Another 12 passengers were seriously injured and taken to Bendigo and Ballarat.
Aerial footage of the area shows a massive effort under way by paramedics, SES and other workers amid debris strewn across a wide area.
The bus came to rest among trees beside the highway and a large section at the top of its rear has been torn off.
It is understood the driver lost control of the bus and rolled down an embankment just after 3pm.
The bus was carrying around 30 adult passengers.
Others suffered minor injuries are were taken to a relief centre in Avoca.
Police say the Sunraysia Highway will remain closed in the area for some time.
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