MyGov is starting to come back online after the portal experienced technical problems — denying people online access to the ATO, Medicare and Centrelink — while Telstra has blamed "unusually large traffic" in NSW for yesterday's payments outage.

Key points: The National Retail Association estimates that Thursday's outage cost retailers $100m

The National Retail Association estimates that Thursday's outage cost retailers $100m The Federal Government's myGov system was also down this morning, affecting people trying to lodge tax returns

The Federal Government's myGov system was also down this morning, affecting people trying to lodge tax returns RBA figures show more people are using electronic payments than ever before

In a tweet announcing services had been restored, myGov said a small amount of people "may still experience intermittent issues logging in as we return to full capacity".

The Australian Government's myGov portal — the online system that millions of Australians now use to access their employer payment summaries in order to file their tax returns, as well as to access Medicare and Centrelink services — went down on Friday morning.

Tax Office services through myGov were also affected, but the ATO said returns were still being processed and paid as usual.

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Earlier, the ATO told ABC News the issue was related to "a technical issue with a communications switch", and was unrelated to the Telstra outage.

"We are working to restore services as quickly as possible," spokeswoman said.

"This is unrelated to the Telstra outage of yesterday and also not as a result of increased volumes of users. It will not have any impact on people who have already lodged their returns."

The ATO, she said, had already processed more than 1 million tax returns, and 390,000 refunds with a value of $882 million had been paid into accounts this morning.

"A further 110,000 refunds worth $292 million will be paid into accounts this afternoon," she said.

"Today, a total of 500,000 refunds worth $1.2 billion will be paid into peoples' bank accounts."

Earlier, a spokesman for the Minister for Government Services, Stuart Robert, had confirmed the system was down.

"Some services, including myGov, are currently unavailable or experiencing slowness," he said.

"The departments are working on the issue and apologise for the inconvenience."

DHS extends reporting deadline for welfare payments

Newstart recipient Graham said he was worried he would be penalised for not reporting his fortnightly job search efforts because of the myGov outage, and feared he would get his payment docked.

The 55-year-old, who lives on the central coast of NSW, said he used to work in IT, transport and logistics and has been searching for jobs since January, but has to rely on Newstart for the time being.

"I called my employment services provider and they told me, 'tough luck, myGov is down — we don't control myGov'," he told ABC News.

"But I am worried that Centrelink imposes penalties and if the system is not available I will be penalised."

A Department of Human Services spokeswoman said that would not be the case.

"Our staff worked hard to rectify the problem as quickly as possible," she said.

"All services are now available, however a small amount of people may still experience intermittent issues logging in as we return to full capacity."

She said an investigation was underway to determine what caused the outage, "but we have already ruled out any possibility of a cyber attack".

"We can also confirm all regular payments were made overnight and were not impacted by this disruption.

"We're sorry for any inconvenience this outage has caused for customers and have put in place measures to ensure they are not disadvantaged by this.

"This includes extending the deadline for reporting employment income until 7:30pm AEST."

She said people without urgent business were encouraged "to wait until services are fully restored to complete this".

"This will allow those with urgent business to access the services they need," she said.

But Jeffrey, 54, who lives on the south coast of NSW, said his pension payment did not appear in his account this morning.

"It's there every second Friday but it wasn't there this morning," he told ABC News.

"I tried to call Centrelink, but every time you ring it cuts out after three rings.

"My rent is supposed to be paid today and Housing NSW would have not got their rent."

Telstra 'sorry' for outage that may have cost retailers $100m

Telstra has apologised to its customers following a national outage on Thursday afternoon, saying it was caused by "an unusually large volume of traffic across the network" in NSW.

The Telstra outage, which occurred for a few hours from about 2.30pm until Thursday evening, took down electronic payments at retailers, including Woolworths and Caltex, as well as many services across the banking system.

Dominique Lamb from the National Retail Association said Telstra's outage could have lost retailers about $100 million in one day.

"Given both the time of day and the businesses affected, the Telstra outage certainly caused a large degree of inconvenience for both shoppers and retailers yesterday," she said.

"As some shoppers would have paid with cash instead or simply delayed the purchase of essential items, such as groceries, it is still a little difficult to ascertain the exact cost to retail sales at this early stage.

"The amount in lost sales could be as high as $100 million for the day, however, hopefully much of it will be recouped by customers simply doing shopping today and tomorrow rather than yesterday."

Telstra said it would consider compensating its business customers.

"Our account executives are focused on continuing to talk with our customers to understand the impact on their business," a spokesman told ABC News.

"Compensation is something we can consider on a case by case basis."

The Telecommunications Industry Ombudsman (TIO) urged customers affected by yesterday's outage to contact Telstra first, but get in touch with the TIO if their matter remains unresolved.

Cyber attacks ruled out by Telstra

Both Telstra and the ATO have faced a number of outages and technical issues disrupting customers and services in recent years.

Twitter also appeared to suffer a worldwide outage on Thursday, hitting its share price. The social media giant tweeted "miss us?" about six hours ago, but it appears the issue has now been resolved.

Asked whether its outage was related to a cyber attack, a Telstra spokesman said, "I can confirm that this is not the case".

He said the outage affected some of Telstra's IP network services, including EFTPOS, ATM and some payment platforms.

"We apologise for the interruption to some data services yesterday afternoon," he said.

"We know how frustrating this was and worked to restore them as quickly as possible."

He said the company was continuing to investigate the exact cause of this issues "but early investigations suggest it was caused by an unusually large volume of traffic across network links in NSW".

"All services have now been restored," he said.

"We're really sorry for the impact and inconvenience to business and their customers."

The problem with going cashless

Telstra's outage on Thursday highlighted potential problems of Australians increasingly relying on a cashless world.

Some consumers that could not make electronic purchases at stores, later found they were also unable to withdraw money from ATMs.

In November, Reserve Bank of Australia governor Philip Lowe said cash will become a niche payment sooner than we think and cheques will be phased out.

The RBA said Australians now make, on average, nearly 500 electronic payments a year, up from around 100 per year around the turn of the century.

Despite the decline in cash use, the value of banknotes on issue, relative to the size of the economy, is close to the highest it has been in 50 years. For every Australian there are currently around 30 $50 and 14 $100 banknotes on issue.

In November 2016, the bank undertook a survey of consumer payments based on 1,500 people, recording around 17,000 day-to-day transactions, which found that, although the share of payments made in cash continued to fall, cash was still used for more than one-third of consumer payments.

Cash continues to be used more often for lower-value transactions — it was, the report said, the most common way of making payments of $10 or less in 2016, accounting for more than 60 per cent of these small payments.

But the share of payments of $10 or less made in cash was 16 percentage points lower in 2016 than in the 2013 survey (33 percentage points lower than in 2007), as contactless card and phone payments took off.

Around one-fifth of respondents did not hold any cash at the beginning of the survey week (compared with 8 per cent in 2013), while around one-third held more than $100, a similar share to the 2013 survey.

Older people were more likely to hold more cash, the report found.

The median respondent aged 65 and over held $95 in their wallet at the beginning of the survey week; more than twice as much as the value of cash held by people in other age groups.