PHOTO: A Telstra user downloaded 994 gigabytes of data on Sunday's free data day. (Supplied)

The second Telstra free data day has set another record for the most data downloaded over a 24-hour period on the company's mobile network.

By the end of Sunday, Telstra customers downloaded 2,686 terabytes（兆兆位） of data, the equivalent （相等的） to 3.4 million HD movies（高清电影）, the telco （电话公司） said in a statement.

The downloaded data was 46 per cent more than the amount downloaded on February's free data day.

Both free data days were given to customers as compensation （补偿）after massive outages across the Telstra network earlier this year.

The same user who clocked up 421 gigabytes（兆） on Telstra's previous data free day exceeded his record, using almost a terabyte's worth of download data — 994GB — in 24 hours.

He told the ABC he used 359GB downloading all 14 seasons of Mythbusters（《流言终结者》）, 191GB on all The Simpsons （《辛普森家族》） episodes and used a comparatively lower 15GB to download the Wikipedia （《维基百科》）database.

The high download figure is well above his usual 6GB Telstra Business Plan but unlike other customers, the user said he had no issues with connection, which was a great speed "from midnight to midnight."

However, the telco had to respond to customer complaints of slow download speeds after some users tweeted to Telstra about cripplingly（原意：伤残的-->弱爆的) slow speeds as more and more users clogged up （堵塞） the network.

Customers also complained that they did not even care about the free data, with the network connection so bad that they could not stream the AFL and NRL games — which are already unmetered on their mobile plans.

Telstra acknowledged there were a few hotspots where "heavy users caused localised congestion" and also responded to user complaints on Twitter.

"There was significant demand for network access yesterday," Telstra tweeted, adding that software changes had been implemented.

Telstra said the previous record of 1,841 terabytes of data reached on the first free data day was exceeded by 4:00pm (AEST) on Sunday.

"We reached the peak network traffic level of the previous free data day by 8:00am and stayed above that level until midnight," the company said in a statement.

Customers used their free data to stream and download movies, binge watch TV shows, run computer updates and install software among other things.

In the lead up to the free data day, the ABC provided some suggestions for what to download on Sunday, including downloading Wikipedia or backing up a photo library to the cloud.

The second free data day was to compensate for the 8 million customers who experienced trouble accessing their mobile network last month.