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.