Mr Williams, 25, is in Kharkviv, a city in north-east Ukraine, and has vowed to stay despite being urged to flee when an invasion was imminent.

Mr Williams, from Kirkstall in south-west Victoria, posted mid-afternoon Australian eastern daylight time.

"We've just woken up maybe half an hour ago via shelling," Mr Williams said on a video on his channel after news broke.

"Russia has declared war on Ukraine. Ukraine is under attack by Russia."

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Mr Williams said he, along with Ukraine residents, were planning their next steps.

"We are taking steps personally in terms of what we do from now," he said. "This is what personally I wasn't expecting."

Mr Williams tells listeners he hears a shell as he is talking to the camera.

"This is the news now - we are being invaded," he said.

Last week Mr Williams said residents were talking about the very real prospect of an invasion.

"At the bar last night the news was trickling down and this is when it became real to a lot of people, including myself," he said last week.

Mr Williams said it was surreal. "It felt like, if you've seen Titanic, when the boat's sinking and the band continues to play," he said.

Mr Williams said he and a fellow traveller had been speaking about whether they should leave when they realised this was not an option for so many.

"This is where these people live and they can't leave, some won't leave - they don't want to, they want to stay and fight for their country and that is something that should be praised."

Mr Williams has also been speaking to Ukraine residents for content on his YouTube channel, Willy Beating Cancer.

On one video he speaks to a number of residents in the border town of Kharkiv.

"Kharkiv will never surrender against the Russian occupation," the resident said. "I love Ukraine and I want to fight for this country."

Russian President Vladimir Putin has authorised military operations in breakaway areas of eastern Ukraine.

Mr Putin has authorised a military operation in eastern Ukraine in what could be the start of war in Europe over Russia's demands for an end to NATO's eastward expansion.