The Government could have to find more than $500 million within the next five years to keep KiwiBank growing at its present rate.
KiwiBank's $9 billion loan book is currently backed by $400 million in Government capital.
Massey University finance lecturer David Tripe says $600 million of new equity will be needed if current growth is maintained and the loan book hits $20 billion.
Even more could be needed if Australian-owned banks divert capital to higher-growth Asian markets and their lending in New Zealand drops.
Prime Minister John Key on Monday reiterated that the Government could consider a partial sharemarket float of KiwiBank.