The incident happened at the Post Office on Houghton Road, Thurnscoe, at around 2.30am on Tuesday, November 20.
The gang, who were all wearing black clothing and may have been wearing balaclavas, left the scene in what police believe was a black Audi A4 estate.
Det Insp Mark Monteiro said: 'This is the second incident of this nature at this location in recent months, though the methods used last time were different.
'It's too early to say at this stage whether the incidents are linked but we've had detectives at the scene conducting thorough enquiries and scouring CCTV footage in a bid to identify those involved.
'Those enquiries are ongoing and I'd urge anyone who may have seen or heard anything suspicious to contact police.'
Anyone with any information should call police 101, quoting incident number 52 of November 20 or Crimestoppers, anonymously, on 0800 555111.