An agreement allowing New Zealand to access information from the Cook Islands if the Government believes tax evasion is occurring was signed on Friday.
The Tax Information Exchange Agreement allows for the full exchange of information on criminal and civil tax matters between the Pacific countries.
New Zealand Prime Minister John Key and Cook Islands leader Jim Marurai signed the agreement in Rarotonga.
The Cook Islands is included in the Organisation for Economic Co-operation and Development's list of tax havens.
During the "Winebox Inquiry" in the 1990s New Zealand companies were accused of using the Cook Islands to dodge tax payments.
Mr Key says the agreement will allow New Zealand authorities to investigate an illegal company or trust set up for the purposes of evading tax.
He says the agreement is part of a worldwide movement led by the OECD and G20 countries to try to eliminate tax evasion worldwide.
Under the agreement, tax officials will be able to request tax records, business books and accounts and bank information from Cook Islands.