France is trying to keep a computer hack of frontrunner Emmanuel Macron's campaign emails from influencing the outcome of the presidential election.

The electoral commission has warned it may be a criminal offence to republish the data.

Macron's team said a "massive" hack had dumped emails, documents and campaign financing information online just before campaigning ended yesterday (NZT) and France entered a quiet period, effectively forbidding politicians from commenting on the leak.

Polls have been predicting that Macron, a former investment banker and economy minister, is on course for a comfortable win over far-right leader Marine Le Pen in Sunday's election, with the last surveys showing his lead widening to around 62 per cent to 38.

"We knew that this kind of risk would be present during the presidential campaign, because it has happened elsewhere. Nothing will be left without a response," French President Francois Hollande told French news agency AFP.