Higher staff cost continued to weigh heavily on transport operator SBS Transit's profitability.

The ComfortDelGro-owned group posted a 30.6 per cent drop in net earnings to $3.16 million for the quarter ended June 30, despite revenue rising by 6.7 per cent to $209.34 million on the back of an ever-growing ridership.

As it ramps up headcount to prepare for the opening of Downtown Line 1 in December, SBS's staff cost rose 12.3 per cent to $93.9 million.

Higher expenses for repairs and maintenance and increased depreciation charges also eroded earnings.

A 4.5 per cent drop in fuel and electricity costs to $43.37 million hardly cushioned the sharply higher expenditure.

Operating margins shrank from 10 per cent to 9.6 per cent in the quarter.

Earnings per share slid from 1.47 to 1.02 cents a year earlier while net asset value per share stood at $1.10, down from $1.11 on Dec 31.

The directors have declared an interim dividend of 0.9 cent, down from 1.35 cents previously.