The bill for the Canterbury earthquakes blew out by 25 per cent to $20 billion yesterday - and it may push to $30 billion or more, the Treasury says.
Most of yesterday's increase, laid out in the pre-election finance report (Prefu), was due to higher estimates of damage to houses and other residential buildings from the February quake.
It also included damage from subsequent aftershocks.
But the Treasury said yesterday's estimates remained an uncertain "working assumption".
It noted the estimate took in damage to property, contents, and infrastructure valued at current prices.
It did not include business disruption or additional costs from inflation, insurance administration or rebuilding to higher standards than existed before the earthquakes.
"These costs are important and their inclusion could lift the total cost to $30 billion or more."
That figure includes private sector costs borne largely by insurers as well as the Government's share, which stands at $13.5 billion.
Finance Minister Bill English yesterday said it was "improbable" the total bill would reach $30 billion but with the Treasury estimating that building cost inflation will hit 7 per cent in 2013 it appears likely that the $20 billion will almost inevitably rise.