The volume sold for September delivery was 31 million litres, compared with 37m litres in August, and 51m litres in May.
The average price was 21.54p per litre, 2p better than last month.
The last time the auction price topped 21p was in September 2008.
“But farmgate prices still continue well below the cost of production,” said Ulster Farmers Union deputy president John Thompson.
Across Europe, butter is now trading at a premium to intervention levels.
Many industry analysts believe world supplies of dairy products are tightening.