In the first post, Elemi wrote "The .co.nz address points to the .com address." with the .co.nz site being the old one that now has trouble to update. The problem might be that the site URL register in the config file is redirected to the other site (other server?) and try to connect to this site database which has different credentials.
The solution would be to remove this redirect. In this case the .co.nz URL won't be accessible until the update is done, this the inconvenient of working on live sites.