The letter also said she made comments about white people to another employee, saying, "If they were Maori they wouldn't be like that."
Ms Harris conceded she had told another employee the customer was arrogant. She offered to write a letter of apology.
However, she denied she had called the customer an "arrogant p****" and said she was not racist.
The Warehouse launched an investigation and found Ms Harris guilty of serious misconduct.
It concluded she did not make any racist comments, but found she did say "arrogant p****".
The ERA found the incident was one of serious misconduct and The Warehouse had carried out a proper investigation.
However, the decision to dismiss her was not fair or reasonable, given Ms Harris' good past track record and the fact she believed she was carrying out her proper duties.
The ERA did not award any remedy because of her contribution to the situation.