That document, signed by representatives from V8 Supercars, Ateed and Pukekohe Park, was shown to the Herald on Sunday. It says V8 Supercars is responsible for any extra costs, which should be met in a "timely manner".
He had no concerns that V8 Supercars would do so if needed, O'Riley said.
"Absolutely, because part of the due diligence we've done is into the nature of their finances and their backers. The risk profile is different [anyway] because we're not the promoter or underwriter, but even so it's just good prudent commercial management."
The Auditor-General is investigating a complaint by councillor Cathy Casey that Ateed hid a secret report from councillors before the vote, while other councillors condemned the organisation for withholding due diligence and risk assessment reports.
However, O'Riley said two workshops were held with councillors before the proposal went to the finance and strategy committee. The first, on May 31, included "robust discussion", but was not attended by councillor Cameron Brewer, who has been one of the harshest critics.
Commercially sensitive due diligence and risk assessment reports were not taken to the meeting because councillors did not ask for them, he said.
"We've got nothing to hide but we do have to take into account private information that sits in some of these reports."
Ateed set up the advisory group, which any councillor could join, and those members had seen the full reports. They have since backed the event.
In future they would ask councillors during workshops if they wanted to see any other documents, O'Riley said.
