Footage of the Qantas jet dumping fuel above Sydney was sent to news.com.au.
The QF63 flight had a crack in the windshield. QF63 passenger Jackson Reynolds said on Twitter there was an issue with the heating mechanism, which caused the crack.
A Qantas customers are also complaining on Twitter about another flight, QF23, which maintenance crews have been fixing for five hours. The issue with that aircraft is unknown.
Qantas said in a statement flight QF7 was also suffering from a mechanical issue.
"The flaps on the aircraft (which are attached to the wing) are unable to retract which means the aircraft can't fly efficiently," the statement said.
"As the Dallas flight is our longest on the network, the captain made the decision to return to Sydney.
"The aircraft - an Airbus A380 - is expected to land at around 4pm Sydney time where it will be inspected by engineers.
"A second flight, Qantas flight QF63 from Johannesburg to Sydney also needed to return to Sydney due to an engineering issue.
"This aircraft has landed safely. It is a Boeing 747-400 and has an unrelated issue to the QF7.
"Our operational teams are working through accommodating passengers or offering them transport home before replacement services are organised."
The latest incident comes after a Qantas flight was forced to land in June after it was dumping fuel at sea.
A Melbourne woman on the Los Angeles-bound flight QF93, which diverted to Sydney said the crew made the decision to turn around.
Jessica McCallum, 29, praised Qantas crew for their handling of the situation, after an emergency light came on in the cockpit of the A380 about an hour into the flight.
"A staff member came over toward our row and asked the people in front if they would mind if they had a look at the engine outside the window," she told news.com.au while still sitting in her seat on the tarmac at Sydney Airport.
"He then rushed off and we didn't hear anything for a while.
"We were then told about the oil leak affecting the second engine on the left side.
"We were told we would divert to Sydney and land in 25 minutes.
"We were circling around for ages until the pilot could get the centre of gravity of the plane level just so we could land safely."
In December 2014 a Qantas flight made an emergency landing in Perth after the plane's air conditioning failed.
It was put into an emergency descent while travelling at some 39,000 feet, eventually levelling out at about 10,000 feet, in order to maintain internal air pressure.
