Part of the 66-acre facility next to Junction 10a of the M20 is being used as a coronavirus test site for lorry drivers hoping to cross the Channel.
Now new drone photos capture just how big the plot is - as staff prepare for an influx of lorries at the end of the Christmas lull.
Ashford MP Damian Green described the move as a "really bad error", but says capacity at the lorry park has been "handled very well so far".
"The number of lorries using it grew last week, and everyone in the haulage industry expects it to be busier this week," he said.
"It's not surprising the drivers are turning up in Mersham.
"Thankfully the village was cleared with police and Kent County Council officers directing them away from the village, but that's a temporary solution.
"I don't know if the lorry park is getting its own postcode, but I also don't know if it needs one.
"All drivers need is a postcode to any surrounding businesses that will lead them to the site, or signs pointing out the entrance."
After starting work in July, contractors are still behind on the build programme as inclement weather is continuing to delay construction.
The government had hoped it could hold up to 1,700 trucks on the plot and use the land for customs checks from January 1.
But heavy rain has meant the checks are being carried out on a temporary facility at the nearby Waterbrook Park estate, with only part of the Sevington site currently available.
The Department for Transport (DfT) is directing hauliers arriving at the Eurotunnel terminal near Folkestone without a negative test result or valid Kent Access Permit to the Sevington plot, which is officially called the Sevington Inland Border Facility.
On the government's website, the incorrect postcode has now been removed.
The DfT says it "strongly advises drivers and crew to get a negative test before they travel to Kent or other Channel crossing points".
It comes as drivers heading to France from the UK have to show proof of a negative coronavirus test taken in the previous 72 hours.
The DfT says: "Drivers and crew using Dover and Eurotunnel will get fast-tracked past queues if they get tested and have a valid Kent Access Permit before arriving in Kent.
"Testing is available at Manston Airport and Ashford Sevington in Kent, but there is a chance of severe delays.
"To avoid delays, get tested at an advice site before entering Kent."
The Sevington site is set to become fully operational in late February.