A Virgin Australia flight headed from Los Angeles to Sydney was forced to turn around after a plumbing problem caused the bathroom to overflow.

Passengers complained of an “unbearable” smell after liquid from bathrooms ran down the aisles and claimed that human waste had seeped into the cabin.

"We were literally sitting in crap,” said passenger Julia Malley, according to The Mirror.

Other passengers, such as Julia Malley, took to social media to complain.

Malley posted this to her Facebook page:

She wrote: "Stuck in LA... 3 hours in to our flight they decide to turn around because the bathroom exploded and we were literally sitting in crap. 2 rows away and offered masks for the 3 hours back. YUCK!!!!!"

Passengers were offered masks and the cabin crew attempted to clean up the mess, but ultimately the flight was forced to return to LAX only three hours into the transatlantic journey.

Virgin Australia described it as a ‘fresh water overflow’ from lavatory sinks and denied passengers were forced to sit among human waste.

"The onboard toilets operate on a completely separate drainage system," officials told the Herald. "As the issue was with the aircraft sink, and not the toilets, there was no incident of leaked human waste."

In a letter to those affected, Virgin Operations Manager Roger Lindeman offered his “sincere apologies” for the disruption encountered.

“Virgin Australia takes great pride in offering a superior product, on time performance and providing customer service excellence. Therefore it is with much disappointment I am writing to you on this occasion,” Mr Lindeman said.

“Unfortunately the aircraft did need to return to Los Angeles due to an issue with the plumbing in the freshwater overflow system on board and could not continue with the journey.”

Virgin Australia said that the passengers were given hotel rooms and were working on getting them on alternative flights as soon as possible.