[nznog] increasing number of blacklisted NZ public IP addresses

Hello all, I'm working for a small web/mail hosting company. I've recently noticed a lot of blacklisted IP addresses from NZ based ISPs being dished out, part of our intrusion prevention methods involve denying connections from these addresses. How do these blacklisted IP addresses get unlisted? Is it the responsibility of the customers of these ISPs or it is the responsibility of the ISP? Daniel Christie