The bus was carrying members of a school brass band that was on a four-week fundraising tour from Tonga.
The band was on its way to Gisborne to perform at a church on Christmas Day, when the bus plunged down a steep bank.
Three people were killed and many others were injured.
The crash killed 11-year-old Sione Taumololo and a 33-year-old Talita Fifita.
Leotisia Malakai, 55, died later from her injuries at Waikato Hospital.
In the Waitakere District Court this morning, Ritchies entered a guilty plea through its lawyer to an amended charge laid by WorkSafe, for failing to ensure the health and safety of other people was not put at risk.
Last year, the driver of the bus, Talakai Aholelei, was sentenced to five-and-a-half months' home detention and had his license suspended for two years.
He pleaded guilty to three charges of careless driving causing death and 27 charges of careless driving causing injury.
He also had to pay $36,000 in reparations.