Cole Miller in an old photo shared to Facebook by a friend.

Cole Miller is being remembered by loved ones and strangers around Australia in a staggering outpouring of grief for the 18-year-old victim of an alleged one-punch attack.

The teenager was punched from behind in Brisbane’s Fortitude Valley on Sunday night and died on Monday afternoon from “massive brain injuries”.

The tributes began to flow after an emotional statement from Cole’s father Steven on Monday, who described his son as a “beautiful, brave young man” with his “whole life to be lived”.

• Brisbane teen dies after one-punch attack
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• ‘One punch changed everything’

Following Mr Miller’s death, Queensland Police announced the two men accused of the incident would have their charges upgraded to “unlawful striking causing death.”

They had previously faced Brisbane Magistrates Court charged with grievous bodily harm. They have been named as Armstrong Renata and Daniel Maxwell, both 21.

In a Facebook post, Mr Miller’s former water polo team the Brisbane Barracudas extended the condolences of the sporting community to his family.

“18-year-old Cole Miller was in the prime of his life, a talented young water polo player reaping the rewards of his hard work and dedication to the sport he loved,” the post said.

Mr Miller was an elite water polo player. Photo: Facebook

“This was his second season playing at the elite level with the National League and he had a promising career ahead of him.”

Friend Javan Haryanto posted a photo of himself with Mr Miller to Facebook captioned: “RIP Brother, I love you man”.

“Rest easy mate, it was a pleasure knowing you” another friend posted on Facebook.

Joe Cook wrote on Facebook: “RIP Cole Miller … I’m glad I got to spend one more Christmas with you. But I would never have thought it would be the last .. I love you mate x. You will be in my heart forever lil cuz.”

The hashtag #ripcole was also started on Twitter and strangers shared their shock and outrage.

A Facebook event “Remember Cole Miller” was set up encouraging people to attend a barbecue in the Brisbane CBD on January 11 as a tribute to the teen.

“Let’s march together in his name and raise our voices together,” it wrote.

Watch Steven Miller’s emotional statement and see other tributes below:

Watch: Steven Miller reads a statement on Cole Miller’s death https://t.co/EFck9ysBZn

— ABC News (@abcnews) January 4, 2016