Western Sydney motorists will feel the difference in traffic as they travel on the newly widened M4 motorway, but in a month's time, so will their pockets.

The NSW government officially opened the newly widened four lanes on the M4 in both directions from Homebush to Parramatta on Sunday, and while it will provide traffic relief for motorists, it comes with a price tag.

From August 15, motorists will pay a distance based toll of between $1.77 and $4.56 following a one-month toll-free period.

Tolls were removed from the M4 in 2010 but the government has justified their reintroduction insisting it's the fairest system.

"People have a free option – if people don't want to use a toll road they can use the existing roads to get to where they need to be if they are price sensitive," Premier Gladys Berejiklian told said on Sunday.

"We also make sure that you only pay for what you use.

"We are making the system as fair as possible and as affordable as possible."

However, western Sydney motorists aren't happy with the new tolls, with some saying they won't be able to afford it and will instead be forced to use the congested Paramatta Road.

"People have a choice between paying $2000 a year in a tolls or travelling on one of the most congested roads in Sydney – it's not much of a choice they're given," Opposition Leader Luke Foley said.

While the opposition claims it supports lane widening on the M4, it did not support the reintroduction of tolls on the road.

Mr Foley insists the toll should be cancelled as motorists are paying for one extra lane and not a new road.

"We have said there is a role for tolls in the overall Sydney roads needs, but it's not the case for a new toll on the M4 that people have already paid off," he said.

The M4 widening is the first completed part of the government's $16.8 billion WestConnex project.

AAP