Malcolm Turnbull has warned a trade deal with Australia is no substitute for the United Kingdom's economic ties to Europe.
The former prime minister has sounded the alarm over the financial implications of the messy Brexit divorce.
"The reality is that half your trade goes to the European Union," Mr Turnbull told the BBC on Wednesday.
"Britain's trade with Australia is a very small percentage, as indeed it is with Canada and other countries in the Commonwealth."
Britain intends to sign a string of post-Brexit trade deals after breaking away from the European Union.
"The problem that Britain faces today is that in an age of rising protectionism, the United Kingdom has chosen to walk out of the biggest free trade area in the world," Mr Turnbull said.
"Its economic prospects, its trade prospects, now depend on cutting new and better deals with a whole range of countries, not least of course is the EU itself."