Combining Wellington and Porirua city councils could mean a 10 percent rates hike, but may help deal with growth.
Wellington and Porirua councils have been in talks about a possible merger for about a year.
A plan was recently presented to councillors by Wellington City Council chief executive Kevin Lavery who said Porirua City Council had initiated the talks.
Deputy mayor of Wellington Justin Lester said the possible merger had been poorly thought out, and would result in significant rates rises.
He said the biggest burden would be on commercial ratepayers, with an increase of about 6.5 percent.
Annual rates hikes would bring the total to about a 10 percent rise, he added.
"For any business that would be very hard to swallow," he said.
However, Porirua mayor Nick Leggett said the merger would help deal with future growth pressures on the region.
"Porirua is not coming with its begging bowl on this," he said.
"We have the same credit rating as Wellington city, we're a much smaller city - we're effectively the size of one of their wards, so, they're much bigger, they have more resources.
"But it's not as though we don't bring something to the table, for instance about half our workforce goes into the Wellington CBD every day."
A Wellington Chamber of Commerce spokesperson said they thought the merger should be explored, and questioned the suggestion that Wellington ratepayers would pay more if a merger went ahead.