Jurors in the hush money trial of former US president Donald Trump finished their first day of closed-door deliberations on Wednesday without reaching a verdict, extending a trial that will decide the fate of the only US president in history to be charged with a crime.

The seven-man, five-woman panel, plus six alternates, are due to return to the New York courthouse at 9:30am local time (11.30pm AEST) on Thursday, but it is unclear when they might reach a verdict.

The jurors are tasked with deciding whether Mr Trump is guilty of any of 34 felony counts of falsifying business records.

Prosecutors say Mr Trump falsified the records to veil reimbursements to his then-lawyer Michael Cohen, who paid porn star Stormy Daniels $US130,000 ($196,000) in the final weeks of the 2016 campaign not to air her claim that she and Mr Trump had sex a decade earlier.

The former president and presumptive Republican nominee has pleaded not guilty, denies any sexual interaction with Ms Daniels and argues the payments to Cohen were correctly designated as legal expenses in company records.

Now that the case has gone to the jury, here's a look at how the deliberations will work.

What is the jury deciding?

If the jury convicts Mr Trump, it must unanimously find he created a false entry in his company's records, or caused someone else to do so, and that he did so with the intent of violating or concealing a violation of a state law making it illegal for conspirators "to promote or prevent the election of any person to a public office by unlawful means".

A conviction would mean that the jurors all agree that something unlawful was done to promote Mr Trump's election. But they don't have to be unanimous on what that unlawful thing was.

In this case, jurors could choose between three possible unlawful acts:

• that one of the conspirators broke the Federal Election Campaign Act, which in 2016 made it illegal for a person to give something worth more than $US2,700 to a presidential campaign;
• that other business records were falsified, including federal tax forms that Mr Trump's company issued to Cohen, bank records associated with Cohen's payment to Ms Daniels, or an invoice used when the National Enquirer's parent company paid Karen McDougal, a former model who also claimed to have had an affair with Trump;
• that false information was submitted on a tax return.

For a conviction, each juror would have to find that at least one of those unlawful things happened, as well as finding that business records were falsified to cover it up.

Where do the deliberations happen?

Behind closed doors in a room reserved for the jury.

While the 12 jury members deliberate, the six alternate jurors, who have also observed the whole trial, will be in a separate space in the courthouse.

If a jury member is unable to continue because of illness or other reasons, an alternate juror will then take that person's place, and deliberations will begin from scratch.

Jurors went into deliberations after getting detailed instructions on the relevant laws in the case, what must be proven in order for them to convict Mr Trump, and their duty to acquit him if the prosecution's case is not proven.

All of the jury members and alternates have had to hand over their mobile phones to court officers for the duration of the deliberation.

What will we know about the jury's discussions?

Very little. The deliberations are secret, though jurors — whose identities are being kept from the public — can have a court officer deliver notes from them to the judge.

The first such note in this case was delivered about three and a half hours into the jury's deliberations, when the jury asked to re-hear some testimony from Cohen and former National Enquirer publisher David Pecker.