Australia and New Zealand have reached a historic agreement yesterday with the signing of a new protocol that will strengthen economic ties between the two countries.

Prime Minister Julia Gillard and New Zealand Prime Minister John Key signed the Australia-New Zealand Closer Economic Relations Trade Agreement (ANZCERTA) Investment Protocol in Wellington.

The signing of the Investment Protocol is an important development in our broader bilateral economic integration and a significant milestone for Australia and New Zealand, the Australian prime minister said.

"It is the last significant piece of the historic CER agreement that has made Australia and New Zealand's economic integration a gold standard around the world.

"The Investment Protocol will deliver substantial benefits to investors in both countries. It will result in lower compliance costs, less red tape and improved legal certainty for investors.

"This means increased trans-Tasman investment flows, more jobs and greater prosperity for people in both countries."

For Australia, the protocol is expected to result in over half of all new Australian investment in New Zealand being exempt from screening. For New Zealand, the protocol will provide the most liberalised access (with the US) to the Australian investment market of any country.

Current accumulated investment between Australia and New Zealand stands at over A$110 billion. Australia is the largest source of foreign investment into New Zealand.