He said police were conducting an “extensive canvas” of CCTV footage in the area and had made a public plea for information.
Anyone who witnessed the incident is asked to report their details online or call 105, referencing number 251202/3058.
Information can also be provided anonymously via Crime Stoppers on 0800 555 111.
The post alleged the girls ran home and alerted their parents, who attempted to chase the van with their vehicle as it sped away.
They lost the van when it entered the Southern Motorway but were able to flag down a police car.
The police confirmed the sisters were receiving support after the ordeal.