Four Corners has jointly taken on one of the most ambitious investigations in journalism history in a bid to get through the front door of law firm Mossack Fonseca — the parallel world of tax havens. But it was no easy feat, writes Marian Wilkinson.

It was already dark when we landed at Panama's main airport and the drive into the capital was disorienting.

As we tore down the highway past a sea of brilliantly lit skyscrapers, I tried to remember the desolate city I came to 25 years ago at the height of the cocaine wars.

Back then, Panama was a world famous tax haven run by General Manuel Noriega.

With the enthusiastic support of Colombia's cocaine cartels he turned Panama into Latin America's laundry from drug profits.

A lot has changed in Panama since then. While the slums are still here, the generals have gone and democracy has taken root. But Panama is still one of the best known tax havens in the world and that is why I came here.

My aim was to get through the front door of a law firm called Mossack Fonseca — a name synonymous with discretion and secrecy.

Sorry, this video has expired Journalists arrive at Mossack Fonseca headquarters

Mossack Fonseca is one of the most active players in the parallel world of tax havens — offshore centres where respected corporations and business figures can temporarily park their money or do deals with more flexibility, tax benefits or secrecy than they can often get at home.

It is the same parallel world used by arms traffickers, drug dealers and political cronies trying to hide their suspicious financial affairs.

Mossack Fonseca is run by Jurgen Mossack — a Swiss tax expert — and his Panamanian partner, Ramon Fonseca.

The firm has 30 offices around the world and its clients have attracted the attention of tax authorities and police on four continents, including — as I found out — Australia.

Eight months ago I had never heard of Mossack Fonseca, but that changed with a phone call from my old colleague in Washington DC, Gerard Ryle — Director of the International Consortium of Investigative Journalists (ICIJ).

That call brought Four Corners into one of the most ambitious investigations in journalism history.

German ICIJ colleague Bastian Obermayer, from the Suddeutsche Zeitung newspaper, had access to a stunning leak of financial documents from Mossack Fonseca.

The source of this leak acted because they believed Mossack Fonseca's work raised serious moral questions.

From my first trawl of the files it was clear there were some seriously troubling characters using Mossack Fonseca's services.

One of the many who jumped out was Rami Makhlouf — the billionaire cousin and crony of Syrian dictator Bashar al-Assad.

We soon discovered hundreds of Australian clients in the files. Many had companies registered in the British Virgin Islands (BVI).

Using a tax haven is not illegal and the clients ranged from respected corporations to targets of the Federal Police tax investigation — Project Wickenby.

The British Virgin Islands is one of the biggest tax havens in the world. ( Supplied: Jason Hubert )

Mossack Fonseca denies wrongdoing

The BVI is one of the biggest tax havens in the world. This beautiful Caribbean territory has close to 500,000 active companies or about 15 companies for every man, woman and child on the islands.

Following the trail to the BVI was fraught. Our requests for work permits were refused. A Canadian colleague from ICIJ was turned back at the BVI's main airport.

We were forced to go in as tourists and even then we were stopped at the border. After we finally got in, we found out our movements were monitored.

But the importance of this story only really hit me when we got to Panama.

Sorry, this video has expired Reporting on the secrets of the super rich

With producer Ali Russell and crew Ron Foley and Geoff Krix, I met up with some 25 journalists from around the world who had been working like us for months.

We swapped notes on our discoveries of a maze of covert financial dealings by drug dealers, former intelligence officers, businessmen tied up in the giant Petrobas scandal that is threatening the Brazilian President, players in the FIFA corruption case and much more.

We all decided we had to confront Mossack Fonseca at their headquarters to try to get answers to the many questions we had about their global activities.

We were blocked by security guards as soon as we arrived. After almost an hour of negotiations, Mossack Fonseca finally responded.

They sent down their PR man with a statement of blanket denials of wrongdoing.

But standing at the entrance to their headquarters I knew the story of Mossack Fonseca would very soon be heard around the world.

Sorry, this video has expired Explainer: Panama Papers

You can watch 'The Secrets of the Super Rich' on Four Corners on ABC iView.