The matter was first raised with Tana on September 16 when an explanation was sought for the transactions.
But Walker said she did not receive an explanation despite following up with him, and referred the matter to auditors Ernst and Young (EY).
"The Mayor has not provided them with the additional specific information to enable them to resolve the queries raised," she said.
"EY has now closed its investigation and has advised that they can take the matter no further and have suggested that the next steps would be to take the matter to the Serious Fraud Office, the Police or the Office of the Auditor General."
Tana said he worked on gathering that information over the weekend after he was told more detail was needed than the explanation he eventually provided on Wednesday last week.
He said he would give the additional specific information to council staff and auditors by 11am tomorrow.
In an email to Tana on Saturday night, Coffey asked him to also provide a copy of that information to councillors.
"We are absolutely in our right as councillors to ask for transparency and your accountability on this matter."
Coffey said Tana was not a victim of "politicking" and questioned why the matter was not resolved when he was first made aware of it on September 16.
"This is a result of not being responsive to the seriousness of the situation."
She shared Tana's disappointment Walker's email had been leaked to media but called on him to focus on clearing up the issue at hand.
Tana told the Herald he would not reply to Coffey's email and said it was inappropriate for her to make the request given it was an election period and both were in the mayoral race.
He would do what was required of him within the process, he said.
"I have nothing to hide, so once the anomaly has been cleared up, I'll be more than happy to provide that information to councillors."
He 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.
Tana said he provided a written explanation to Walker last Wednesday maintaining he never filled up another vehicle with the petrol card assigned to the mayoral car.
He told her requests made on the same day for him to attend functions or meetings sometimes weren't added to his diary, so it wasn't always an accurate reflection of the kilometres he travelled.
He travelled more hours than council staff because as mayor it was important for him to meet with people face to face, he said.
