A trail of false invoices, tax evasion and evidence tampering ended today in Wellington's High Court for accountants Barrie James Skinner and David Ingram Rowley.

The pair were found guilty of an elaborate scheme to avoid tax, one which embroiled many of their clients and which also saw them attempt to "coach" witnesses and to tamper with computer records.

They were refused bail after Justice Steven Kos found them jointly guilty on more than 100 counts of fraud, tax avoidance and attempting to pervert the course of justice.

Rowley was acquitted on 14 charges and Skinner on nine.

Although both have surrendered their passports, prosecutor Dale La Hood said this did not remove the risk of them skipping the country.

"Any assurances that these men have given about complying with anything has to be taken with a grain of salt," he told Justice Kos. "They are men of means."

Justice Kos refused bail but indicated he would consider a more detailed application.

The guilty verdicts came after a nearly two-month trial in which most of Skinner's and Rowley's clients not only gave evidence against them but included evidence that the pair had tried to coach them to give contrary evidence, if not to the court, then to Inland Revenue.

A scheme involving false invoices and a trail of "shell" entities was set up form September 2005, the invoices being for services which were never provided or, if they were, were provided at a value much lower than that recorded on the invoice.

"This scheme was sold to willing and, in many but not all cases, credulous clients of the accuseds' accounting and taxpayer practice as a means of reducing the amount of tax payable by them," Justice Kos said in his ruling.

When Inland Revenue began taking an interest from late 2009, the two accountants coached clients to give different, and what were hoped to be more convincing, explanations for the invoices.

"Instead of the services described on the invoices, it seems the clients had aquired interests in real property, such as car park licences and apartments within the Wellington central business district, or other contractual obligations necessitating payment.

"This came as news to all but two clients and the evidence of those two I find unreliable... I find the new explanations have no evidence in fact."

Most of the clients, when served with a compulsory conference order by Inland Revenue, had not complied with suggestions from the two accountants about what they should tell the investigators. 

"Contrary to plan the clients told the truth to Inland Revenue when interviewed. Those who did not did not make convincing liars," Justice Kos said.

Most made a voluntary disclosure to the IRD and either have or are in the process of settling those debts.

The judge also found Skinner and Rowley had tampered with a computer records only a few days before the trial began in an unsuccessful bid to change the date at which various invoices and other records were created.

The pair will be sentenced on August 17.