This volume of Christmas mail is double what postal workers would normally handle outside the festive period.
And to deal with demand, an extra 300 seasonal workers have been employed at the city's mail centre alone.
"Everything's on a greater scale," said operations manager Andrew Colclough.
"Not only do we actually need additional seasonal workers in the plant, we need additional vehicles to service the road network.
"We need then obviously more people on the postal element of it, actually delivering that final mail.
"All of the different parts of the operation need to be joined up every single day to make sure you and me get our mail the very next day after it's been posted."
On Thursday morning, hundreds of postal workers had collected mail from postboxes across the Cardiff and Newport areas, bringing it back to the Penarth Road depot.
By the afternoon, the cards, letters and parcels were unloaded from the Royal Mail lorries into the mail processing room - where there are about 760 workers in total.
These items are then placed into the "tumbledryer" - also known as the IMP (Intelligent Mail Processing) machine - which sorts them into first and second-class postage.
From here, they filter through to the ILSM (Intelligent Letter Sequencing Machine) which sorts them into postcode areas and then into "walk areas", specific streets or roads.
Any cards, letters or parcels which have incomplete addresses and missing post codes can be hand sorted.
Mr Colclough said there is an air of festivity around the mail processing room.
"We have lots of different types of people who are really into Christmas. Around the plant we have Christmas trees, lots of joy and festive music, which makes it a really happy time to work."
Royal Mail has warned customers not to get caught out as the last posting date for Christmas delivery is 20 December for second-class and 21 December for first-class.
And for those who really leave it last minute, there is a special delivery option up until 23 December.
More than two million cards, letters and parcels will have been sorted at Royal Mail's depot in Cardiff by the end of today - its busiest day of the year.