All World Cup 2018 matches for the remainder of the group stages will be broadcast on SBS as well as streaming service Optus, with the latter service also made free to all Australians until August 31.

Customers who have signed up to Optus and paid $15 for the service to cover the period of the World Cup will also be refunded after technical issues plagued the telco's coverage of the world's largest sporting event in its opening week.

"We as Optus have tried hard to deliver an exceptional viewing experience and deeply regret this has not been the case for all Australians," Optus chief executive Allen Lew said.

"Since Monday we have continued to improve our product and Optus have delivered the last six matches of the world Cup without issue. This has provided us with the confidence that out efforts have worked.

"As confident as we are in our capabilities, we have listened to feedback from the Australian soccer fans."

Mr Lew was first forced to apologise for the problems on Sunday afternoon, but, after the technical issues persisted, on Monday announced that all World Cup games would be simulcast on free-to-air SBS for two days while it worked to fix the service.

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The telco cut a 2016 sub-licensing deal with SBS, which holds the rights to this and the next World Cup, that allowed Optus to screen all 64 matches at Russia 2018, 39 of them exclusively.

SBS was scheduled to broadcast the other 25 free-to-air, including all Australia matches, the semi-finals and final, but now looks likely to have access to all matches because of Optus's troubles.

The failure of Optus to immediately correct problems whereby viewers were left staring at error message screens instead of the matches prompted outrage among disgruntled subscribers.

What many football fans got instead of a World Cup match when streaming via Optus. ( Supplied )

Optus came under intense scrutiny to fix the issues, to the extent that even Prime Minister Malcom Turnbull personally intervened to put pressure on Mr Lew.

Mr Lew said the cause of the problems centred on "a critical failure in one element of our content delivery service", insisting the company had learned from the issue and was working to "make sure our network is more resilient".

Optus has broadcast English Premier League fixtures for the last two seasons, but this year marks its first foray in to coverage of the biggest international sporting event.

The company's reputation has been badly hit by the fallout from the technical failings, with Optus previously putting them down to a level of "unprecedented demand" for its service beyond its expectations, an excuse that has done little to placate angry customers.

Optus also suggested that most of the problems were with customers using Apple devices and on broadband networks other than its own.

Coverage of the group match between Costa Rica and Serbia was affected by the failure of the Optus streaming service. ( AP: Vadim Ghirda )

For the World Cup, Optus made coverage available to anyone willing to pay for a subscription rather than just customers on its own mobile and fixed line networks.

A decision on which matches will be shown on which network beyond the group stages of the World Cup will be made in the coming week, with Mr Lew saying that those decisions had not yet been made, independent from the technical problems.

The free access to the Optus service extends beyond the World Cup and takes in the first three rounds of Premier League fixtures.