Ice has closed State Highway 8 between Tekapo and Twizel, after two car crashes on Monday morning.
St John were alerted to the first crash at 8:15am after a car went off the road. A man has serious injuries and a woman has moderate injuries.
They were flown to Christchurch Hospital by the Westpac Rescue Helicopter.
An hour later, on the same road, another car crash left five people with minor injuries.
New Zealand Transport Agency (NZTA) journey manager Lee Wright says once the sun had a chance to warm the road, it would reopen.
In the meantime drivers should take an alternative route or delay their journeys until later in the day. Ms Wright says chains should be carried on the alpine passes.
The NZTA says all drivers should be wary driving around shaded areas on South Island roads. Contractors would grit the roads wherever possible.
Ice and fog are making roads hazardous further south around Dunedin as well.
Drivers can keep up-to-date through NZTA's traffic information page or phone 0800 4 HIGHWAYS.