A Belgian student on a summer job has discovered a hidden trove of gold worth an estimated €9 million (£8 million) while digging trenches for a sewage system, causing a stir in the country, reports infohub.kz.

18-year-old Kobe said that while excavating a trench for a sewage system, he saw something in the ground. "At first I thought it was €1 coins. Then we saw a gold bar. It turned out to be a bit more than we thought," he told VTM television.

The discovery took place on the site of a former brewery in Sint-Gillis-Dendermonde, about 30 km north-west of Brussels. The student was working with colleagues. The hoard was stashed in a bag in a bricked-in cellar wall beneath the grand villa originally built for the brewery's owner, Theophilus Van Assche, in the late 19th century.

The hoard includes gold coins, nuggets and numbered bullion bars. Kobe said it appeared to have been deliberately hidden: "It was really bricked in there – not in a safe or anything."

Brewing on the site began in 1899 and ceased in 1970. The property is being completely renovated by the social welfare organisation CAW Oost-Vlaanderen. Geert Hillaert of CAW said labourers were laying drainage pipes when they stumbled across the gold.

Kobe said they immediately called the police and notified CAW. The treasure has since been transferred to a secure facility in Brussels pending an official investigation to establish its legal ownership. Hanne Ollevier of the East Flanders public prosecutor's office said authorities must first verify that the bullion was not stolen property or the proceeds of criminal activity.

Belgian law requires finders to wait at least five years for potential owners to claim any treasure. The building contractors, the welfare organisation and any descendants of the once-wealthy Van Assche family could all have a claim. Piet Buyse, a former mayor of Dendermonde, said he was confident that the Van Assche fortune had "certainly stayed in the family" and that it wouldn't surprise him if they had hidden some of it in gold.

Kobe said he and his colleagues had understood straight away that they had to hand over such a big find. "If it had been a few coins, maybe not," he said. "But maybe we'll get some kind of finder's reward eventually."