The Fair Work Ombudsman is launching legal action against two companies over the alleged underpayment of twelve trolley collectors in Sydney.

The Ombudsman Nicholas Wilson alleges 12 trolley collectors, mostly South Korean nationals, are owed more than $27,000.

It is alleged the men, aged between 19 and 32, were not paid over an eleven-day period while working at the Costco shopping centre at Lidcombe, in Sydney's west, in July 2011.

The employees were allegedly entitled to be paid up to $3,000 each but received nothing.

Mr Wilson says the significant amount of money involved and the fact that the workers were vulnerable with little English was a key factor in taking the action which is yet to be heard in the Federal Magistrates Court.