A council spokeswoman said that "for a range of reasons" the information WDC had about the number of SUIPs on each property had not kept pace with how the buildings were used.
"Sometimes owners have combined previously separate units into fewer units, or have created more separate units since the information was initially gathered."
But the Cameron St property owner said the layout of her building, particularly upstairs where she had small business tenants, had not changed since she inherited it in the 1980s.
Thirty of the 373 commercial premises reviewed by WDC had gained five or more SUIPs.
Councillor Shelley Deeming - who heads WDC's Finance Committee said the system had been changed to make it "fairer all round".
"...There will be some ratepayers who feel disadvantaged and some who feel advantaged - and we want it to be fair to all. For several years [some] have been charged less than their share of the commercial rates. That is unfair on other ratepayers, and we want to fix it."
She said council had recognised that this year's bill had caused "some pain for a number of ratepayers", thus a review of WDC's rating policy was scheduled for the 2016/17 annual planning process.