Westpac Banking Corp was last week given a near-billion-dollar tax bill, which many believe may have ramifications for the cost of credit in New Zealand.
The Australian bank lost a court case against the Inland Revenue Department (IRD) that wiped 0.25% off its parent's Tier 1 capital ratio.
Mr Truman expected Westpac to appeal the decision, even if the bank believed it did not have a chance of winning.
However, what concerned Mr Truman the most was the lack of rules around tax avoidance, even with the IRD winning with Westpac and an earlier similar case with the Bank of New Zealand.
To date, the Supreme Court had only issued judgement in one major tax avoidance case - Ben Nevis - at the end of 2008.
The judgement in the Westpac case was the third major High Court case since to rely on Ben Nevis.
The first - Penny and Hooper - found for the taxpayer.
The BNZ case, and now Westpac, were found to have fallen on the avoidance side of the line, he said.
"While the cases slowly progress through the courts, Inland Revenue has been careful not to clarify the essentially grey area of tax avoidance.
"A draft interpretation statement designed to clarify the the tax avoidance boundary for taxpayers is long overdue."
It was rumoured that the interpretation statement was being held up by the Crown Law office, which principally wanted to to focus its energies on litigating the structured finance cases without distractions, Mr Truman said.
The release of a second successful structured finance judgement would no doubt be celebrated by both Crown law and IRD.
Hopefully, both could now finalise a release of the long-awaited guidance for taxpayers on tax avoidance.
Asked what the lack of rules meant to his clients, Mr Truman said people with "genuine" commercial transactions looked at the tax implications for how the deal was structured.
People would be "extremely cautious" on commercial transactions because of the lack of certainty from the IRD.
"In terms of certainty, the expectation is that there will be a greater incentive to use binding rulings by taxpayers in relation to more issues and more transactions than what may have historically been the case and more than what may actually be optimal."
But even binding rulings could be uncertain if the IRD decided to act more conservatively about what was acceptable, he said.
A binding ruling was when a business went to the IRD, set out all the information and the IRD provided a binding ruling on the tax implications.
Mr Truman expected the Westpac case to make it harder for businesses to get a ruling and it was possible that later, the IRD could still challenge the transactions.
One of the positive outcomes from the Westpac and BNZ cases was the possible $2.2 billion of revenue going to the Government, he said.
"This is a huge win for the Government and its tax income."
It meant the Government might not run such a large deficit, it might not have to borrow as much overseas as planned and it would have more income to spend on things like tax cuts.
But it could be two or more years before the money was received by the Government, if the appeals went ahead.
"The issue, ultimately, is about having legislation that works and which taxpayers can navigate through with appropriate certainty," Mr Truman said.
