Sims said the transfer of accounts between the computing platforms went "quite smoothly" with only a few technical issues cropping up.
These included a small number of ATMs being offline yesterday morning that are now back up and running, Sims said.
"There were little technical things but we're happy that yesterday we were able to bring up all of our channels. That means our online banking, our call centre, the IVR [Interactive voice response] systems customers can use their phones for and our branches were ready for opening this morning," Sims said.
"But first week you always remain vigilant to make sure things go well," he said.
While IT systems have been switched and online banking branded to ANZ, National Bank branches will be progressively rebranded over the next two years.
"Progressively from this week staff are in the new ANZ wardrobe. If you go to National Bank branches whilst from the outside of the branch you'll see the old green facade, internally you'll start seeing ANZ promotional material because from today onwards we're presenting ourselves as the ANZ, the physical changing and painting of the branches it's a two year program," Sims said.
Information about the switchover at the ANZ's site here.