Tana has previously said it had been a difficult time for his family recently with his auntie and best friend passing away.
The school holidays, the mayoralty and running a campaign had also taken up his time, he said.
The issue was then escalated to the Office of the Auditor General on September 27 and on their advice auditors were engaged to undertake an independent investigation.
On October 2 Tana provided reassurances that the card had not been used improperly, but did not provide details to explain the unusually frequent transactions, Walker said.
After Tana missed a further deadline of October 4, Walker made the call to contact police and councillors were advised on a confidential basis, she said.
"I felt that Mayor Tana's non-response made escalation necessary. I am employed by the Council to ensure public money is spent carefully and within well prescribed limits.
"It was certainly not my intention that the matter would be leaked to the media. I regard that as unfortunate and both myself and Mayor Tana have suffered ill-informed public comment as a result."
After eventually receiving further information, auditors found the unusually frequent transactions were due to personal use of the mayoral vehicle accounting for 64 per cent of the period covered in the report.
Walker said she previously explained to Tana that the Remuneration Authority calculates personal use at 20 per cent of overall use and that he needed to contain his use to this proportion.
Tana objected to the report being released last week leading into local body elections, but said it cleared his name.
Walker said she wanted to release the report last Thursday so it could speak for itself.
She said she had not received a formal request from Tana to undertake an investigation.
"I have undertaken an internal review of the email traffic surrounding the leak of information and can confirm that there was no release of my confidential email to councillors from staff. I don't have any practical way of checking councillors' emails. My duty as Chief Executive is to treat all candidates in exactly the same way. My obligation was to follow a fair and reasonable process. I think I have done this.
"I have felt constrained in what I can say publicly but feel a need to clarify these matters now that there has been misinformation in the public domain. I have also contacted Mayor Tana to outline these issues."
Tana has previously said he deserved an apology over the whole saga and his name should be fully cleared.
He said the time period covered in the report included days where he picked up his children and maintained he had full private use of the mayoral car.
Tana has been approached for comment.
