Emergency services have started looking at waterways as they expand the search for a British backpacker who hit his head before running away from his campsite early on Saturday morning.

Key points: It is believed Aslan King "may be disoriented and lost in the bush"

It is believed Aslan King "may be disoriented and lost in the bush" Police said they remained confident they would find Mr King alive

Police said they remained confident they would find Mr King alive His friends told police his behaviour on Saturday morning was out of character

Aslan King, 25, was camping with friends near Princetown, about 230 kilometres south-west of Melbourne, when it is believed he had a seizure and hit his head about 2:00am.

His four friends, also British nationals, told police he then suddenly got up and ran from the area into the surrounding bushland.

"They're very concerned for his welfare," Acting Senior Sergeant Travis Barber said on Sunday.

"It was out of character for him to do this, and [they] have never actually seen him behave in this manner before."

Acting Senior Sergeant Barber said it was believed Mr King "may be disoriented and lost in the bush".

Mounted police have been part of the search effort. ( ABC News: Sarah Jane Bell )

The search was expanded on Monday to include waterways and all areas within a four-kilometre radius of the campsite.

Mr King had been holidaying in Australia for two weeks and had left Melbourne on Friday to head to the Great Ocean Road.

Acting Senior Sergeant Barber said the travellers had only arrived at the campsite on Old Coach Road on Friday and would be "very unfamiliar with the area".

The group was camping near the world-famous Twelve Apostles. ( ABC News: Andrew Gray )

The campsite, about 7 kilometres from the popular Twelve Apostles, sits near the Gellibrand River and is within hundreds of metres of the ocean.

Specialist bushwalking teams were called in to assist Victoria Police, the State Emergency Service and Parks Victoria in the search.

Duncan Brookes from Bush Search and Rescue Victoria said his team was helping to comb through the coastal scrub.

"We've been asked to probe up the hillside for any open areas where a person might have been able to travel," Mr Brookes said.

"It's difficult, there's open areas and then very thick areas … it's not suitable for line searching."

The friends were camping near the south-western end of the Great Otway National Park. ( ABC News: Sarah Jane Bell )

Sergeant Danny Brown said on Monday police remained confident they would find Mr King alive.

"As much as the terrain is harsh, if he wants water he can find water, so we've got those basic survival [needs] there," Sergeant Brown said.

"We're not looking for any foul play, we're looking for a missing person.

"We maintain the thought process that we're looking for [is] a person that would be in some sort of condition that would need help."

According to his social media, Mr King is an illustrator usually based in Bristol.

Sergeant Brown said Mr King's friends were "as bewildered as most of us" about why he left the campsite.

"They're worried, they look concerned, their friend's gone missing and has been missing for three days," he said.

Mr King's family has been informed, however police said they had not heard whether they were making their way to Australia.

Police said Mr King was wearing trackpants, a light grey T-shirt with a grey hoodie and green runners and may possibly have blood on his forehead.

Anyone who may have seen the 25-year-old is urged to contact Warrnambool Police Station on 5560 1333.