This was the smallest rate of decline so far this year, though sales were down 1% when compared with July.
Retail sales figures this year have been badly affected by a sharp fall in sales in the motor sector.
These dropped by almost 30% in the year to August. When these are excluded, the volume of sales was down 5.2% over the year.
A breakdown showed that bar sales are down more than 13% over 12 months, while categories linked to the property sector - furniture and lighting, electrical goods and hardware, paints and glass - are all more than 11% lower.
The value of sales, which takes prices into account, fell 13.4% compared with August last year, with a monthly drop of 1.4%, indicating that retailers are still lowering prices in a bid to attract customers.
