(Adds details on total sales, background) April 29 (Reuters) -...

(Adds details on total sales, background)

April 29 (Reuters) - Australia's second-biggest grocery chain Coles Group Ltd (COL) said on Wednesday supermarket sales rose 13.8% in the third quarter, boosted by a buying frenzy driven by the coronavirus pandemic and the subsequent lockdown.

The company said "unprecedented demand" due to customer stockpiling began in late February and continued throughout March, further strengthening after social curbs were imposed.

As the pandemic spread beyond China, people flocked to supermarkets and stocked up on essentials, causing March retail sales in Australia to spike the most on record, with supermarkets selling an extra A$2.2 billion worth of goods.

Coles said sales at its supermarket division grew to A$8.23 billion in the quarter ended March 29, from A$7.23 billion a year earlier.

The supermarket division, which accounts for a majority of the company's earnings, logged same-store sales growth of 13.1%, surging from 2.4% last year.

Total sales, including those from its liquor business and the company's fuel and convenience stores, grew 12.9% to A$9.23 billion, beating a Citigroup estimate of A$9 billion.

The retailer, however, warned it expects higher costs during the fourth quarter because of investments being made due to the outbreak, such as paying for extra staff and higher spend on cleaning, hand sanitizer products.

Coles and larger rival Woolworths Group (WOW) were forced to hire thousands more to meet the growing demand for supplies and to help enforce distancing measures as a result of the virus outbreak.