Tower has secured more reinsurance cover in case another catastrophe occurs in the current financial year.
The insurer says it has cover in place for two more events in the year to end of September.
It also recalculated the expected costs of February's earthquake and last month's aftershock.
Including the cost of extra reinsurance, it is expecting expenses to rise by between $22 million and $26 million in the year.
Previous estimates put the costs at between $22 million and $31 million.
Shares in Tower have risen two cents to $1.50.
