More than 18,000 low-income seniors across the country had monthly benefit payments suspended last year when the Canada Revenue Agency didn't process their tax returns on time.

Three of the four Atlantic provinces — Newfoundland and Labrador, Nova Scotia and New Brunswick — were particularly hard hit by the problem.

They accounted, combined, for nearly 6,300 of those cases.

Internal CRA briefing materials obtained by CBC News shed new light on what caused the issue, the impact it had, and what federal officials plan to do to avoid it from happening again.

Guaranteed income supplement benefit

The benefit in question is called the guaranteed income supplement, or GIS. It supplements the Old Age Security pension.

To keep receiving the GIS every year, seniors must have their annual income confirmed, to ensure it remains low enough for them to qualify. That usually happens through their tax return.

The deadline to file taxes is the end of April. For the payments to keep flowing, CRA has to assess those returns by the last few days of June, and forward the information to Employment and Skills Development Canada, which sends out the cheques.

Last year, that process worked for most people — the vast majority of the two million seniors entitled to get the GIS.

For thousands of others, it didn't.

Susan Miller told CBC News last summer that the $600 per month she receives from GIS makes up a large portion of the income she and her partner share. (CBC)

According to government briefing notes, Employment and Skills Development Canada "began receiving calls — seemingly concentrated from the east coast — from some GIS recipients whose payments had been suspended but who had advised ESDC that they had submitted their tax returns on time."

Susan Miller of Chester Basin, N.S., was one of them.

Miller told CBC News last summer about the impact on her and her partner. She said they rely on the monthly payment.

When it didn't arrive as usual in the final week of July last year, she said, "the bottom fell out of my world."

"I have a certain amount of money, and I pay my bills, and what's left over is for us to survive on for the month. That $600 means I'm going to have a problem."

Miller recently told CBC News that her situation was resolved within weeks of going public.

CRA briefing materials indicate all 18,000 affected had their accounts processed by late August.

Tax info processed after deadline

Last summer, federal officials tried to find out what was going on, and why people like Miller weren't getting their benefits.

Among the factors they identified:

CRA didn't have time to reprint and redistribute pre-printed envelopes for paper returns, to change the address to the new processing centre, causing "delays between five and 10 days."

Late filing patterns meant a "higher than normal influx of returns" right at the tax filing deadline.

Reminders mailed out to seniors by ESDC "may not have generated the necessary actions by the GIS recipients in time."

To fix the problem, CRA officials indicated they would ensure all pre-printed envelopes would have the correct address in the future. Nearly all of the tax returns in question were filed by paper, not electronically.

Almost all of the 18,000 problematic tax returns last year were filed on paper, not electronically. (Chris Young/Canadian Press)

The briefing notes also suggested that processing speeds would get better because of economies of scale offered by larger centres.

And there were meetings planned between CRA and ESDC officials to figure out more ways to reduce any "disruptions."

'Significant measures' put in place

CRA did not make anyone available for an interview, but stressed that a number of actions have been taken to address the problem.

"Any individuals who miss out on receiving their benefits is a concern," the agency said in an emailed statement.

"This is why the CRA has put in place significant measures to ensure that GIS recipients who file on time get their benefits without interruption."

Those include:

automatic enrolment for newly-eligible individuals for the GIS;

the creation of a tracking process during peak filing season that will identify GIS-eligible files, and ensure returns are processed in priority;

cross-referencing a list of eligible GIS clients from ESDC to double-check if their tax return has been assessed.

CRA also says it has made it easier, in general, for people to file their tax returns this year.