Sales fell 10.8 per cent in September compared with the same month last year, a steeper decline than the revised 5.4 per cent drop in August.
Excluding motor vehicles, sales shrank 12.7 per cent.
On a month-on-month seasonally adjusted basis, retail sales dropped 4.5 per cent, reversing the 1.8 per cent rise in August.
The larger decline in September was mainly attributed to the computer and telecommunications equipment industry, where sales fell 22.9 per cent, SingStat said. The sector recorded lower sales of mobile phones compared with the high base in September last year when there were new mobile phone launches.
Other industries also recorded double-digit drops. The sales of food and alcohol tumbled 41 per cent, while takings at department stores shrank by 39.8 per cent.
The sales of medical goods, cosmetics and toiletries slid 30 per cent, while sales of wearing apparel and footwear also declined, by 28.4 per cent.
But supermarkets and hypermarkets continued to prosper, with sales increasing by 17.9 per cent.
Sales of furniture and household equipment also rose, by 10.9 per cent.
Takings at food and beverage services fell 29.9 per cent in September, deepening the slump of 28.1 per cent in August.
This decline was led by food caterers, which saw takings plunge 78.2 per cent as demand for event catering remained low.
Sales at restaurants dropped by 33.1 per cent, while cafes, foodcourts and other eating places had takings fall 17.6 per cent.
The total sales value of food and beverage services in September was estimated at $629 million. Of these, online food and beverage sales made up an estimated 20.4 per cent.
Overall, the estimated total retail sales value in September was about $3.2 billion. Of this, online retail sales made up an estimated 11.2 per cent.