A "community build", hosted by his group Brick Expo Canberra and held at National Museum of Australia (NMA) on Monday, allowed the children of Canberra to get hands-on with a Lego mosaic set to be framed and displayed in the Canberra Hospital's paediatrics unit.

"The hospital can be quite clinical, with white tiles and white walls, and it was an opportunity to liven it up with some colour," Mr Boddy said.

"We're Lego creators, we produce artwork, it's very colourful and it's a perfect fit."

The Lego event was shrouded in mystery as the final 55,000-block picture was kept a secret from its contributors. Each of them built only a 5cm x 5cm square based on a pattern before it was carefully added to the mural by volunteers and progress live-streamed to online viewers.

The completed three-panel mosaic, now revealed, shows a clownfish among seaweed and will feature as part of the Canberra Hospital's "art therapy" program.

"It fits in with the hospital's 'nautical' theme. A lot of the art therapy around the paediatrics part of the hospital is…underwater themed," Mr Boddy said.

He said the community event proved popular among Canberrans because Lego is "a drawcard" and more than 3500 people were estimated to have joined in.

"Whether they're adults or children, Lego is still very popular, and we find that it crosses over so many demographics. It is a good way to get people together."

Sarah Gray, from Scullen, and her "Lego-obsessed" 5-year-old son, Seth, came along because he was excited to see and help build the Brick Expo mural.

"And we did explain to him that it was for sick children and he thought that was a great idea," Ms Gray said.

"I think it is fantastic, too. Seth himself has been in that hospital quite a lot of times and to see Lego there, or something made of Lego, would just have made such a difference and made him smile occasionally."

The Brick Expo Enterprises and Canberra Lego Users Group are regular contributors to the Canberra Hospital Foundation, raising more than $250,000 over the last six years to purchase pieces of medical equipment for the Centenary Hospital for Women and Children at Canberra Hospital.

Now in its seventh year, the not-for-profit Brick Expo will be held in Canberra in August, attracting Lego enthusiasts of a variety of ages to view epic cities and hundreds of unique models and miniatures made from the tiny plastic bricks.