A report from the Fair Work Ombudsman has found that over the past year the number of underpayments of trolley collectors decreased to five from six the previous year.

The value of underpayments decreased to $17,170 from $40,441 the previous year.

Coles is subject to an enforceable undertaking from the FWO after an investigation found rampant underpayment of trolley collectors hired through businesses contracted to perform that task at its supermarkets.

A 2015 audit of 15 trolley contractors to Coles found 245 employees had been underpaid $112,000.

Supermarket rival Woolworths has also been subject to an investigation by FWO regarding the payment of trolley collectors by third parties.

Fair Work Ombudsman Natalie James said "Coles deserves credit for cleaning up its act".

"During the second year of our EU with Coles, the number of supermarkets with in-house trolley collection services has increased to almost 85 per cent of all Coles sites," Ms James said.

Coles had set itself a target of bringing all of its trolley collection services in house by end of September 2016.

The EU will continue until December 2018, but will cease beforehand if Coles reaches 100 per cent in-house trolley collection.

A spokesman for Cols said the company had transformed its trolley collection service and now directly employs 3100 to collect trolleys.

Of Coles 776 existing stores, 111 still use contractors but Coles expects that to change in the coming period.

"Coles is continuing to roll out this direct employment model which replaces the previous system where Coles had around 30 contractors engaged to provide trolley collection services.

We will continue to work with the FWO under the terms of the enforceable undertaking," the spokesman said.