Ms Rose-Innes says she now has no excuses to not to write her next book
Henrietta Rose-Innes has told the BBC her prize-winning short story is an "apocalyptic vision" of how polarised South Africans would react in a crisis.
Ms Rose-Innes won the $20,000 (£10,000) Caine Prize for African Writing for Poison at a ceremony on Monday night.
It is set in Cape Town as people are forced to flee an ecological disaster.
With the iconic Table Mountain obscured by toxic clouds, some leave the city late and end up stranded outside, forced to interact with one another.
"It's kind of an apocalyptic vision - but the core of the thing happens outside the city at a petrol station where everyone who's been fleeing the disaster ends up," the South African writer said.
"It is not so much about the explosion - on a deeper level, it's a breakdown of traditional social divisions and social groupings.
"Cape Town is an extremely diverse population with a lot of different communities historically that don't always interact with each other."
The other four writers on the shortlist were from Ghana, Malawi, and Nigeria and South Africa.
The prize, given for a short story, can be awarded to African writers who have published fiction within Africa or elsewhere in the world.
The prize is named after Sir Michael Caine, a former chairman of Booker plc.
Last year's Caine Prize was won by Ugandan writer Monica Arac de Nyeko for the Jambula Tree, a story about lesbianism.
'Lost her way'
Announcing the prize, Judge Jude Kelly said Poison, published in the anthology African Pens, was a "strange disorientating metaphor for leaving it too late... [when] something is coming towards you and will destroy you".
"The narrative employed imagery that pushed the story forward and it showed a sharp and rare maturity," she said.
Ms Rose-Innes said the protagonist of Poison, Lynn, was a version of herself - a middle-class woman in her thirties who has "lost her way".
Unlike the other characters, Lynn is unable to accustom herself to the changing world and is left eating crisps at the garage as the others leave.
"She can't stay in her old ways, symbolised by Cape Town which is now under threat, but she's also not quite ready to take that step into the broader reality of the country with its dangers and opportunities," Ms Rose-Innes told the BBC's Network Africa programme.
Ms Rose-Innes is relatively experienced compared with recent winners.
She has already had two novels published as well as editing an anthology of South African writing.
Her novels Shark's Egg and The Rocket Alphabet have been well received - she studied creative writing under the Nobel laureate JM Coetzee.
Winning the prize guarantees more interest from publishers and reviewers as well as including a month's scholarship at Georgetown University in Washington DC.
"What the prize gives you is time to write, the most precious commodity for a writer," she said.
"It also gives me no further excuses for not writing that next novel, which is good for me."