He has apologised to Mr Ryall for the error, which was pointed out by former board chief financial officer Ewan Soper last week after he saw the briefing paper Mr Thomson presented to Mr Ryall on the $16.9 million fraud.
Former board information technology manager Michael Swann and his friend and business associate, Kerry Harford, are awaiting sentence in the High Court at Dunedin in March, after being found guilty on fraud charges late last year.
Mr Thomson is under threat of dismissal by Mr Ryall, who has said the fraud has affected his confidence in the governance of the board and its leadership.
In the briefing to the minister, Mr Thomson, on the basis of information provided by regional finance officer Robert Mackway-Jones, had stated the actual IT spending had risen from $4 million in 1999-2000 to $5.1 million in 2000-01 and $7.8 million in 2001-02.
(Boards were established at the end of 2001 and the first meeting of the new board, with Mr Thomson as chairman, was in February 2002.) Mr Soper took issue with the figures as presented.
After Mr Mackway-Jones reviewed them, he discovered a change of accounting methodology had taken place between 2001 and 2002, of which he had been unaware.
Mr Thomson said this related to the way IT depreciation and leases were accounted for and where they appeared in the accounts.
"When this accounting change is factored back in, then the IT spend in the first two years would be $7.6 million in 1999-2000 and $8 million in 2000-01."
Mr Thomson said the figures in the briefing paper for the following years, when he was the chairman, were accurate.
They showed $7.96 million spent in 2002-03 (budgeted amount $7.5 million), $8 million in 2003-04 (budget $7.8 million), 2004-05 $7.7 million (budget $7.2 million) and 2005-06 $9.5 million (budget $9 million).
His conclusion that no significant increase in IT spending had occurred during his time as chairman remained correct.
In his paper to the minister, Mr Thomson said the spending only increased in the year Swann came under suspicion (2006).
No significant unexplained variances against the budget had occurred that might have prompted further concern.
Mr Soper yesterday said he was satisfied the figures that had now been sent to Mr Ryall were correct and more reflective of the trend at the time.
The changes to accounting practices had occurred when Healthcare Otago and the Health Funding Authority merged into the district health board, he said.
