Bulgarian customs officials have seized 712kg of heroin from inside two Iranian trucks entering Bulgaria from Turkey.

Officials on Friday valued the record heroin haul at NZ million.

The drugs were found in two Iran-registered trucks at the Kapitan Andreevo checkpoint on the Turkish border.

They were carrying construction materials and were bound for Austria.

Haskovo region prosecutor Ivan Stoyanov said the heroin was hidden in sealed bags stashed between the officially declared goods.

The two drivers, both Iranian citizens, were detained and could face up to 20 years in jail if convicted on drug trafficking charges.