Federal agencies were instructed to prepare hours before the deadline, as the pay of hundreds of thousands of federal workers will be affected.

Here is exactly what that means. 

When does the shutdown start?

The shutdown started at 12.01am today (2.01pm AEST).

The duration and consequences of the shutdown are uncertain and will largely depend on how swiftly Democrats and Republicans can negotiate an agreement.

Previous shutdowns lasted an average of eight days. 

What makes this shutdown different from the others?

Trump has indicated he might use the shutdown as an opportunity to cut the federal workforce, instructing federal agencies to create plans for extensive layoffs if a deal isn’t reached.

He said it could include “cutting vast numbers of people out, cutting things that they like, cutting programs that they like”.

This threat coincides with more than 100,000 federal workers poised to leave their positions after participating in Trump’s deferred resignation program aimed at reducing the federal workforce.

It is shaping up to be one of the largest mass resignations in US history. 

Will a shutdown affect travel to the US?

No, immigration and airport security are deemed essential services and will continue to run throughout the shutdown.

It should not affect any Australians travelling to the US.

However, there may be limited federal services available while visiting the US.

— With Associated Press