He said the WDHB's revenue was about $240 million.
The forecast deficit for this year was not particularly large compared to other deficits, he said.
"The reason that we're forecasting that at this stage is that the amount of patients going out to other district health boards for care is greater than what we have allowed for in our budget."
Patients would go elsewhere for care when WDHB did not provide services for them, Mr Walden said.
Another reason for the forecasted deficit was the "growth in aged care", which fluctuated year to year and could be hard to predict, he said.
As well, in some areas the WDHB experienced increases in demand which are not catered for in the budget.
"The benefit side is more patients are getting care, but from a fiscal point of view we need to achieve this break even, and the board is certainly looked at by the Ministry [of Health] to ensure that we do work to achieve a break even."
Mr Walden said WDHB had made "some good gains" with ACC revenue.
"We haven't given up on achieving break even at the end of this year. Remembering the figures we're talking about is a forecast. The figures could moderate, especially across the summer period. So the forecast will change as we go forward."
Mr Walden said management wanted to signal to the board that a deficit was forecast, even if there was a chance there might not be one.
"There's no good simply announcing we're not going to break even with only two months to go," he said.