Watch the moment a pod of killer whales appear to free a humpback in waters off Western Australia's south coast

The humpback appears to be heavily infected with whale lice and tangled in ropes, according to Whale Watch Western Australia's Gemma Sharp.

Ms Sharp was onboard the tour vessel that spotted and filmed the encounter off Bremer Bay on January 10.

An injured and restricted humpback whale would usually be an easy meal for a pod of killer whales.

"It would usually be part of their diet so when we saw the injured humpback and the approaching orca, we were concerned," Ms Sharp said.

Witnessing a humpback whale swimming in Western Australian waters at this time of year is a particularly rare sight.

Believed to be a juvenile humpback, it would usually be feeding in Antarctica in January.

"Lifting his fluke we could see that this humpback whale was entangled and our hearts sank, a terrible situation to be in when trying to defend yourself against the oceans apex predators," a statement from Whale Watchers Western Australia said.

The humpback was quickly approached by at least three orca, known to the touring company as Hookfin, Blade and Matriach Queen.

"He rolled and during this process we could see Blade swimming directly underneath his fluke and looking at the rope entanglement.

"Further orca arrived in the area and similar approaches were made as they charged towards the humpback while he defended himself with pectoral fins and fluke swiping.

"Matriarch Queen arrived and moved towards the humpback whale which caused a commotion of white water and then something incredible happened, a large chunk of the green rope that was entangling this humpback floated free behind him."

Having been freed, the humpback then swam away from the pod, who had apparently "lost all interest" in their potential meal.

"Once the orca had left he approached us, circling closely around our vessel which gave a perfect opportunity to observe the severity of the injuries from the rope and it was good to see the vast majority of the line was no longer on this whale anymore."