Registered Democrats living in the Australian capital cast their vote in the party's presidential primary on Saturday, helping to decide whether Hillary Clinton or Bernie Sanders will challenge the Republicans for the top job in November.

Expatriate Americans who wanted to declare either "I'm With Her" or "Feel the Bern" had to be registered with the Democratic Party's official overseas arm, Democrats Abroad, in order to cast a ballot at the voting centre in Smith's Alternative in Civic.

The local voting centre was the first established in the ACT since Democrats Abroad opened a Canberra chapter last July.

Other voting centres were set up in Sydney and Melbourne for voters to make their choice, with the Democrats Abroad primary running globally from March 1 to March 8.

Democrats Abroad is allotted 17 of the 4764 delegates who will decide the party's candidate for president at the Democratic National Convention, which will be held in Philadelphia in late July.

The group's ACT chapter chairman Mike Heffron said many Americans signed up to Democrats Abroad in the weeks leading up to the primary.

"We don't have huge numbers in Canberra, but we had a good turnout considering it's a small chapter," he said.

"Small cities will often remain dormant between elections and then come back to life during election years."

Voters chose their nominee, then pinned their name in their home state on a map of America at the voting centre.

Nancy Opdyke, originally from New York but registered in Michigan, voted for frontrunner Hillary Clinton.

"I've been a supporter of her for a long time," she said.

"We haven't been able to vote in the primary before, so it makes you feel like you're participating and engaging in the process."

At 18 years old, Patrick McGlynn's primary vote was the first he has cast, and the first of four he is eligible to make this year as a dual Australian-American citizen.

"I've cast my vote for Bernie Sanders; I don't think the platform Hillary Clinton is running on is the one I want the country to be run by," he said.

"I think Bernie Sanders is trying to make healthcare better and make life better for students, which is in my interests."

Curious Australians were able to vote in a straw poll at the centre for their preferred presidential candidate from the Republican, Democratic and Libertarian parties.