Staff "not from New Zealand" employed on a major publicly-funded roading project were told they were expected to work Waitangi Day, which an employment lawyer says may breach their human rights.

Engineers working on the $850 million Transmission Gully motorway north of Wellington, a public-private partnership involving the NZ Transport Agency, were sent an email on Wednesday discussing who could have Waitangi Day off.

"It is my expectation if you are not from [New Zealand] then all [e]ngineers should be in work tomorrow and use this day to catch up," the email read.

"If you are from [New Zealand] and Waitangi Day is just another public holiday then I would appreciate if you are also at work.

"Those of you from [New Zealand] who feel it is important to have the day off then please enjoy your time off."

A screenshot of the email was sent to the Herald from a concerned former staff member, who said workers were fearful of speaking out.

The 27km four-lane motorway is being built through a public-private partnership, the Wellington Gateway Partnership (WGP), with CPB Contractors and HEB Construction sub-contracted to carry out the design and construction.

An NZTA spokesman confirmed to the Herald the email had been sent by CPB Contractors.

Employment law expert Max Whitehead said he was shocked at the wording of the email.

"That is so badly worded, seems to discriminate against people not from New Zealand, and gives the connotation and slant there would be a negative impression if they requested not to work.

"It also seems an insult to us Kiwis - Waitangi Day is a special day for us, and we'd like people coming here to be able to enjoy it with us."

All workers were entitled to have Waitangi Day off unless it was explicitly stated in their employment agreement they could be required to work, Whitehead said.

However, to discriminate against workers because of their race, ethnic or national origins - which includes nationality or citizenship - would be a breach of the Human Rights Act.

"There may be requirements in their contracts to work Waitangi Day, but that is no way to go about speaking to staff," Whitehead said.

NZTA senior project delivery manager Andrew Thackwray said the agency recognised the importance of Waitangi Day for everyone in New Zealand, and they were "disappointed" by the email.

"It is important to us that anyone working for, or on behalf of, the [NZTA] is able to observe and celebrate Waitangi Day should they choose to, and we are addressing this directly with the contractor."

A spokesman for the Ministry of Business, Innovation and Employment said anyone working as an employee in New Zealand was covered by employment law, whatever their country of origin.

An employee could only be required to work on a public holiday if this was a condition of their employment agreement.

If an employee worked on a public holiday they would be entitled to payment of at least time and a half for the hours worked.

If the day worked was also an otherwise working day then they would be entitled to a full paid alternative holiday to use at a later time.

A spokeswoman for the Human Rights Commission said if an employee believed they had been discriminated against by an employer on the grounds of national origins then they could lay a complaint to the Commission.