The Canberra official will fly out on Saturday to Rio de Janeiro, where he and his assistants, Matthew Cream from Adelaide and Melbourne's Hakan Anaz, will be based for the duration of the tournament.

A school teacher by trade, Williams has taken unpaid leave from Belconnen High School for the first semester to put all his focus on the World Cup.

He is one of just 25 referees at the tournament and will not find out which or how many of the 48 group games he will officiate until two days before each match.

His performances during the group stage will dictate how many games he gets and whether he progresses to the knockout stages of the tournament.

"If you don't do well in your first game then you can go home,'' Williams said.

"One decision can end a career.

"This dream is the culmination of 22 or 23 years of work as a referee.''

Williams was kept busy during the A-League season and was also on the panel for the Asian Champions League, when he officiated a game in Saudi Arabia in front of 41,000 fans.

He has a long association with both of his assistants, working with Cream since 1995 and Anaz since 2002.

The trio have been working together as a crew for the past three years.

They can be assigned any game at the World Cup except those featuring teams from Group B, which includes Australia, Spain, the Netherlands and Chile.

They are also unlikely to be given games involving teams from the Asian Football Confederation.

All of the referees will be based in Rio de Janeiro until they fly out for games, which are spread across the country.

"We will be in camp for two weeks before the World Cup begins on June 12,'' Williams said.

"The first focus will be on recovery, then will we do work in the class room to make sure we're up to speed and also training on the field.

"We will also study the different teams and the different scenarios, because it's a case of 'expect the unexpected'.''

Williams was grateful to Belconnen High School and the ACT Department of Education and Training for allowing him to take unpaid leave to chase his World Cup dream.

"I've popped in a couple of times and everyone's been fantastic,'' he said.

"All the students have given me a lot of support, and I can't thank the school and the education department enough.

"It's a bit strange to see the fortnightly pay slip come through with all zeros, but it's a sacrifice that's definitely worth it for a once-in-a-lifetime opportunity.''