Tony Abbott's florid defence of Brexit and call for a "one-page" free trade deal to be signed with the UK is part of a "cunning" strategy to undermine Malcolm Turnbull, the Opposition says.

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"It looks like a pretty deliberate strategy," Opposition spokesman for trade Jason Clare told AM.

"Malcolm Turnbull's on holidays and so Tony Abbott is filling the gap."

Mr Abbott's thoughts on Brexit were contained in a foreword he wrote for a report published by the Free Enterprise Group, an influential Conservative Party think tank in Britain.

"Brexit means that Britain is back," he wrote in comments widely reported in the British media.

"The country that gave the world the English language, common law and the Mother of Parliaments is once more to seize its destiny as a global leader."

He also questioned the future of the European Union, writing "... the remaining members of the European Union will now have to rethink how much of their sovereignty they wish to surrender".

Mr Clare characterised the comments as, "Tony Abbott giving gratuitous advice to people all around the world but also to the Australian Government and Malcolm Turnbull in particular about what he should be doing".

"He's as cunning as an outhouse rat, Tony Abbott, he knows what he's doing here and it's bad news for Malcolm Turnbull," he told AM.

He also accused Mr Abbott of mounting a strategy to return to cabinet if there were a reshuffle, a move which would mean he would need to curtail advancing unauthorised positions on behalf of the Government.

"You'd expect that at some point Malcolm Turnbull would need to come back from holidays and do the right thing and sack Sussan Ley," Mr Clare said.

The Free Enterprise Group report urges the UK Government to focus on Britain's ties with Commonwealth countries after Brexit is complete, in around two years' time.

Tim Hewish, who co-authored the report, helped convince Mr Abbott to write a foreword for it.

"We wanted to get someone from the Commonwealth of significant stature," Mr Hewish said.

"We thought because Tony Abbott's got a commitment to free markets and was a former prime minister, he would be the prime candidate."