Kiwibank's internet banking exports transactions as CSV, and its files are a classic case of the debit/credit split that accounting imports choke on. Here is how to export and convert to Xero, QuickBooks or OFX with the signs handled correctly, entirely in your browser.
In Kiwibank internet banking, open the account, use the search or download area, set your date range, and choose CSV. As always, test a short range first and compare a few rows against your statement before importing a long history.
Kiwibank exports commonly split money into separate credit and debit amount columns and include a running Balance column, plus payee and reference fields for the other party. In the converter, assign the money-in column to Credit and the money-out column to Debit, leave Balance unmapped, and map the other-party name to Payee. If your file instead has one signed Amount column, map that alone. Dates are day-first.
A running Balance column should never be mapped as Amount: if your imported figures look like account balances instead of transaction values, that is what happened. Map the credit and debit columns instead.