Lanes Group withdrew 200 gallons of water from the device in West Bromwich, Sandwell, on September 30, 2019.
The firm also took water from a hydrant in Tamworth, Staffordshire, on March 17 last year.
The company pleaded guilty to two offences at Dudley Magistrates Court after a successful prosecution was brought by South Staffs Water, which manages water networks in the region.
The West Bromwich incident took place on West Bromwich Street at the junction with Roway Lane.
South Staffs Water said Lanes Group, which is based in Leeds, took 200 gallons of water without saying why it was needed.
Lanes Group offers drainage and waste-water emergency services, from removing blockages to repairing drains and sewers.
Mike Morris, network performance manager at South Staffs Water, said: “When companies illegally connect to the water supply network, the quality and the pressure of water supplied to our customers can be compromised and that is not acceptable.
“Providing a safe and reliable water supply to all of our customers is our number one priority and we work hard every day to make this happen.
“We take it very seriously when the actions of others impact our ability to do this, so are pleased that this prosecution has taken place.
“When companies need water, they can hire an authorised standpipe to connect to the water mains network.”
The other offence took place on Clifton Street, Tamworth.
Lanes Group admitted both offences at Dudley Magistrates Court on February 22.
In response to the story, a Lanes Group spokesman said: "Lanes Group plc takes these matters extremely seriously and acknowledges that on these two occasions incorrect standpipes were used.
"We acknowledged these errors at the earliest opportunity.
"As a national contractor, we have crews which operate across multiple water authority networks, sometimes two or three depending on geographical locations.
"Therefore these crews have to carry multiple approved and licensed standpipes, for use within the various authority networks.
"Human error regrettably resulted in the use of the wrong standpipes on these occasions, for which we were fined £500 per incident, plus costs.
"We have carried out additional refresher training on the importance of using the correct standpipes relative to the location.
"Lanes Group spends hundreds of thousands of pounds every year on the hire of relevant standpipes, paying South Staffs Water in the region of £150,000 per year in regards."