The Auckland Council needs to find an extra $300 million over eight years for a new Super City computer system.

Auckland ratepayers have already paid $56 million for a computer system to support day-to-day functions, such as payroll, email, telephone and recruitment.

That system was built by the Auckland Transition Agency, which had the job of melding the eight councils into the Auckland Council.

Now the Auckland Council must untangle the complex web of all the other processes and systems inherited from the previous eight councils.

Only then will it be able to send out a single rates bill and have one system for managing and paying suppliers.

The cost of building a new computer system over the next eight years is $506 million - $56 million already spent, $150 million of budgeted spending and $300 million of unbudgeted spending.

Councillors voted on Wednesday for an extra $43.2 million in this year's budget to start addressing the $300 million shortfall. They will have to find an extra $86.2 million and $77.8 million in the following two years.

Senior council officers said a single new computer system was necessary to improve service to customers, reduce the risk of system failures, boost IT efficiency and capacity, meet regulatory and legal requirements, and produce savings.

Officers looked at continuing with the existing system, but found it could not meet future requirements and would lead to significant operating costs.

Auckland Deputy Mayor Penny Hulse, who has taken a strong interest in the costs of the new system, said she knew amalgamation would be very expensive and she would be keeping an eye on the costs.

Before the Super City election last November, she spoke of the need "to be honest with the people of Auckland about the real costs associated with the Super City".

The $56 million bill for the first Super City computer system showed ratepayers paid $53.8 million for the Auckland Council system, but only $2.2 million for a similar system at the mega-transport agency.

The officer in charge of overhauling the council computer system, Mike Foley, said the two organisations and their needs were quite different.

The transition agency decided to largely rebuild an existing system from scratch for the Auckland Council because of the complexity of the system needed to serve 8000 staff.

The Auckland Transport computer system was copied off existing platforms to serve 1000 staff.

An evaluation by Ernst & Young recommended a new computer system for the Auckland Council - a view shared by the Department of Internal Affairs' technology arm.

But concerns have been raised in IT circles that the transition agency could have copied existing systems used by Auckland City, Auckland Regional Council and Waitakere City Council.

Former regional council IT head John Holley said the complexity of the two systems was not the issue, especially as transport was more complex in some areas than council work and vice versa.