Published: 14th September 2009 | Author: BBC News
It became stranded around 40 miles (64 km) away at Cwm Pennaner, between Frongoch and Cerrigydrudion, Conwy.
The lorry belongs to the Stobart Group, which said the agency driver was "inexperienced". It apologised and said it hoped to remove the lorry later.
"The driver has driven three miles down the unclassified road before coming stuck in mud," said Farmers' Union of Wales officer Gwyn Williams.
He said similar errors happened too often and advised drivers to use a map instead.
"It appears that the driver has followed the instructions on the sat nav," said Mr Williams.
He said that after coming off the M6 the lorry has been directed to Bala and then told to turn at Frongoch, and then on to the unclassified road for Cwm Pennaner.
"This kind of thing happens far too often and we, as an union, are asking drivers to ignore sat navs, use common sense and read a map," he added.
"It will be very difficult to get the lorry out," he said.
The Snowdonia Society said there was concern about the "common practice" of "juggernauts rat-running" through the National Park on unsuitable roads, rather than using the A55.
Society director Alun Pugh said: "We have raised this matter with officials at the Trunk Roads department.
"GPS technology is well-proven and it is quite possible for manufacturers to program their machines to avoid sending huge trucks through narrow lanes where they pose a serious safety hazard."
The lorry, belonging to the Stobart Group, was being driven by an agency driver.
A spokeswoman for the company said: "The driver was an inexperienced and incorrectly coded his sat nav.
"We apologise for any disruption caused by this incident."
The company currently has people at the site, and they hope to move the lorry later on
