It's no secret that many wealthy Canadians are squirrelling away fortunes offshore to avoid — or even evade — taxes.

What is secret is just how much money it's costing fellow Canadians and the national treasury each year.

That's because unlike many other countries, Canada fails to disclose or even track the full size of its "tax gap" — the difference between the government's potential tax revenue and what it actually manages to collect.

The U.S. has been tracking and publicly reporting its tax gap for more than 50 years. And now so do more than a dozen other Western countries, including the U.K., France, Germany, Australia, Sweden, Portugal, Mexico, Norway and Denmark.

The Canada Revenue Agency isn't just keeping data from the prying eyes of journalists.

The agency won't share what it does know with parliamentarians either.

"It's shameful," said Senator Percy Downe of P.E.I. "The Canada Revenue Agency is the most incompetent department ... in the government of Canada."

Downe spoke to CBC News and the Toronto Star, partners in the Paradise Papers collaboration with the International Consortium of Investigative Journalists, which has shed light on the activities of thousands of wealthy individuals and corporations around the globe who use offshore havens to shield money from tax collectors.

Gap campaign

For five years, Downe has been campaigning to get the CRA to release raw data on just how much potential revenue slips through tax collectors' hands due to legal tax avoidance and outright evasion.

"We don't know — is it $40 billion, $6 billion?" said Downe, pointing to a 2017 Conference Board of Canada report that concludes the country's tax gap could be as big as $47 billion a year.

Senator Percy Downe of P.E.I. wants the Canada Revenue Agency to track the country's annual tax gap. (CBC)

Downe says knowing the size of the tax gap is important for two reasons. The first is a matter of principle: Canadians need to feel as though they're participating in a system where everybody is contributing their fair share. The second reason is practical: billions of extra dollars could be put to good use.

"There's lots of things the government could be doing … retiring debt, lower taxes, fund new programs," he said. "It goes on and on."

In 2012, Downe recruited an important ally in his fight: Parliamentary Budget Officer Kevin Page, the watchdog of the country's finances.

Page asked the CRA for raw data so he could conduct an independent calculation on behalf of Parliament to estimate how much in offshore, GST and income taxes the CRA fails to collect each year.

The request prompted a bizarre, protracted struggle with the federal tax agency.

CRA Commissioner Andrew Treusch wrote to Page in March 2013 to question the merits of his request.

"As you know, the CRA does not generate information or data on the tax gap, and there is much debate about the precision, accuracy and utility of any methodology to calculate a tax gap," Treusch wrote.

In February 2014, the agency objected to the request over concerns about sharing private data of Canadian taxpayers. Jean-Denis Fréchette, the new parliamentary budget officer, pledged to abide by strict privacy agreements and even offered to seek the blessing of the Federal Court to ensure it was all legal.

But by August 2014, the CRA argued it would only turn over some limited data and demanded the PBO pay $141,000 to process the request.

The two sides have yet to reach an agreement on the need to systematically track Canada's tax losses.

"We still don't have a global tax gap number," Fréchette told CBC News.

He acknowledges calculating Canada's tax gap would require extensive resources — especially to review and calculate Canadians' offshore holdings. But he says it's an essential measure for the public and elected officials to evaluate the fairness of the tax system and the health of the country's finances.

"Collecting the money is as important as spending the money, or maybe even more important," Fréchette said.

What to do with $6B?

Unofficial estimates of Canada's tax gap vary wildly.

The portion lost to legal offshore tax avoidance, as well as tax evasion, is believed to be at least $6 billion a year, according to the most conservative estimates.

So, what could the government do with an extra $6 billion?

"It's a heck of a lot of money," said Judy Wasylycia-Leis, a former NDP MP. "That might be enough to repair our crumbling infrastructure and sewage plants in Winnipeg, and to invest in a modern transportation system."

Wasylycia-Leis is among those calling on Ottawa to not simply study the tax gap, but to publish the findings and hold the CRA's feet to the fire for failing to collect billions every year.

"With the kind of money we've lost because of offshore havens, we could build great cities."

CRA takes small steps

National Revenue Minister Diane Lebouthillier and the CRA declined CBC's requests for an interview, but in a statement, the agency said it has "begun work" to study the tax gap issue more closely.

After the revelations from last year's Panama Papers leaks and scandals involving KPMG's Isle of Man tax schemes, Lebouthillier pledged in January to take a much closer look at aggressive offshore tax avoidance and launch a "comprehensive study" of how Canada might calculate its overall tax gap.

The CRA has also begun releasing some snapshots of its past tax revenue losses, including its failure to collect $4.9 billion in GST/HST from businesses and $8.9 billion in domestic income tax from individuals back in 2014.

But the agency has yet to determine how much potential tax revenue is lost because of offshore tax havens — be it through outright tax evasion or through aggressive, yet legal, offshore planning.

The government says it is instead focusing on enforcement and has invested nearly $1 billion over the past two years to step up enforcement against offshore tax cheats and promoters. The CRA credits the previous Panama Papers leak with prompting 123 fresh audits and several criminal investigations.

Sen. Downe says while the CRA does a good job cracking down on domestic tax evasion, including posting the names of convicted cheats on its website, the agency does "a terrible job" scrutinizing and policing Canadians' offshore activities.

"Why is that, and why is the gap not being measured as a tool to assess how big the problem is?"