An Australian engineer has been sacked from an NZ power company after running up $NZ23,500 in roaming charges on his work phone on a trip back to Sri Lanka and then turning up for work a day late.

The Employment Relations Authority has ruled the Mighty River Power company was justified in sacking the man but says it must pay him $NZ6000 because of the stress he suffered during the disciplinary process, which had defects.

The Sri Lankan-born man has name suppression until next month.

According to the ERA ruling, he was sacked by Hamilton-based MRP in January after being recruited from Australia six months earlier.

It said racking up a $NZ23,500 phone bill, poor driving after just two to three hours sleep and being absent without authorised leave were serious enough offences to warrant dismissal.

In December, the man returned to Sri Lanka for three weeks with his family to attend a wedding and took his work phone with him.

He only sent a couple of work emails and used data on some apps. His international mobile calls cost nothing, seven international roaming calls cost $NZ5.60 and seven international texts cost 34 cents.

Spark said the remainder of the charges, of around $NZ23,350, were listed as being for international roaming.

The phone used 1GB of data.

ERA member Robin Arthur initially didn't believe such a small amount could have cost so much, but was reassured it was the result of high international roaming charges.

Things were made worse because the man's wife had booked his flight to return to New Zealand the day after he was due back at work. When he got back to Hamilton he drove a work car to the Ohakuri dam, about 120km south, on little sleep.

The ERA said he didn't have to pay back the $NZ23,500.

(AAP)