Emergency services, including an ambulance, two fire appliances and police, attended the scene about 10.30pm.
The female passenger (22) was treated for a minor leg cut by St John paramedics and her boyfriend, aged 27 - who was driving - suffered minor facial lacerations.
A police spokeswoman said the couple are resident in Queenstown.
He said the car's "footing went" as he was trying to negotiate past an oncoming vehicle on the narrow unsealed road.
He said the couple had been on their way to Moke Lake camping ground to "chill out" when the car began slipping from the road before rolling once and dropping on its tyres to the lake more than 2m below.
The car quickly began taking on water. As it was sinking, he said, the pair swam out through windows.
The man said the incident was "scary" and the pair felt lucky not to have sustained more serious injuries.
It is not known whether the driver of the other car stopped, or if either driver will face charges.