Service station owner Ampol has warned of a $100 million hit after the profit earned on converting a barrel of oil into petrol and diesel plunged 83 per cent in one quarter.

Shares in the Sydney-based company dropped as much as 5 per cent on Tuesday after it revealed that its refining margin dived to less than $US1.50 a barrel in the September quarter, compared to almost $US20 a barrel at the same time last year.