Outlandish Treasury forecasts for Kiwibank's mortgage lending have been challenged by the bank itself and raise questions over its future, says Labour.

Budget documents last week contained forecasts of growth in Kiwibank's mortgage lending that are wildly at odds with its recent record and its own expectations.

The state-owned bank currently has loans and advances - mostly mortgages - of about $10.9 billion after growth last year of about $1.9 billion. Treasury's forecast is for growth of less than half that this year, at $836 million.

Over the next four years, Treasury predicts growth ranging from the somewhat high figure of $2.2 billion to the incredibly low sum of $86 million.

Adding to the confusion, Treasury forecasts growth in deposits at Kiwibank will match growth in its mortgages dollar for dollar over the five-year forecast.

A Treasury spokesman said the numbers were taken from data supplied by Kiwibank and its parent, NZ Post, to the Crown Financial Information System and would have been "reviewed by senior people" at both organisations before they were entered into the system.

"It's their best estimate at the time we said 'give us your numbers'."

The very low figures for the final two years were because Kiwibank did not provide forecasts for those years but gave figures which would maintain the "status quo" in its mortgage lending.

The deposit numbers did not come from Kiwibank, however, but from Treasury itself, which made the assumption that any growth in KiwiBank's mortgage lending would not be funded by the government.

"The story we're trying to tell here is that this is fiscally neutral growth."

But a KiwiBank spokesman said Treasury's numbers and their source were "a mystery" and represented "an illogical growth path for the bank".

"We anticipate growth of around $1.5 billion in our mortgages each year and we don't anticipate that that will change.

"We do anticipate that growth in both mortgages and deposits will be constrained through competition and economic conditions that are prevailing, but no matter what, they don't match these figures."

What's more, the dollar-for-dollar match between new mortgage lending and deposits, "is outside the way in which the bank operates".

Labour's state owned enterprises spokesman Clayton Cosgrove said, "Is it the case that somehow KiwiBank will be shedding its mortgage book? Selling off its mortgages?

"The Government's said they're going to sell assets. Is this part of the deal under the radar?

"Somebody needs to come out from under the cover of darkness and explain if this part of the sell-off deal."