Heavy rain led to access problems at Seaclose Park, Newport, which meant thousands of people were forced to sleep in their cars overnight.
Police escorted some motorists stuck in traffic to Newport Football Club where shelter and refreshments were provided.
About 55,000 people are expected at the three-day event on the island.
The problems come as various parts of the UK are braced for heavy rain and flooding. The Met Office has issued yellow warnings for various parts of England and Scotland and a more severe amber warning for the North West of England.
Met Office forecasters said the island would see very windy weather, with sunny spells and scattered blustery showers over the weekend.
Alerts are in place for the Rivers Dyfi, Mawddach and the Dysynni in Gwynedd and Ceredigion.
Up to 70mm (3in) of rain is expected on the hills and mountains of Snowdonia.
The forecourt of a petrol station at Llandre near Bow Street, Ceredigion, was pumped out by Mid and West Wales Fire Service on Thursday night.
Extreme localised flooding caused devastating floods in parts of mid and north Wales earlier this month.
Intermittent heavy rain over the past three days has left many rivers running unseasonably high.
A Met Office heavy rain warning remains in place for north and west Wales.
The Environment Agency Wales flood alerts include areas around the river Dyfi, from Dinas Mawddwy and Llanbrynmair to the estuary, including Machynlleth.
There are also alerts for areas around the rivers Mawddach and Wnion, from Fairbourne to Ganllwyd and Rhydymain and for areas around the river Dysynni from Tywyn to Minffordd.
BBC Wales weather forecaster Sue Charles said: "We're experiencing heavy rain on and off for the best part of three days.
"There are concerns about localised flooding, because of the cumulative effect of this rain falling on saturated ground and into swollen rivers, particularly through parts of Ceredigion and Gwynedd."
People living in the North West of England are being warned over the risk of flooding as heavy rain is forecast for much of the day.
The Met Office has issued a severe weather warning for parts of Greater Manchester, Merseyside and Lancashire.
Up to 50-60mm (2-2.4in) of rainfall is expected and in some places that could rise to 100mm (4in).
The rain could lead to surface water flooding of roads and land, the Environment Agency said.
There is also a risk of floods from rivers in built-up areas which are quickly affected by large amounts of rainfall.
The government agency has urged people in Greater Manchester, Liverpool, the Wirral, North Cheshire and Lancashire to be prepared for the possibility of flooding from Friday afternoon through to Saturday.
Environment Agency teams are set to go out checking flood defences, clearing river blockages and monitoring river levels to try to reduce the risk of flooding.
People are being warned to stay away from swollen rivers and not to drive through floodwater.
Heather Stott, BBC weather presenter in the North West, said: "There will be heavy and prolonged rain through the day, difficult driving conditions and the risk of localised flooding.
"It's going to feel pretty windy too. Gale force winds gusting to 50mph will make for a blustery day.
"The wet and windy weather will continue through the day, clearing all but Cumbria and north Lancashire by the end of the night.
"It will be wet and windy at times on Saturday - but much more showery, with more heavy and persistent rain during Sunday."
This post has been edited by skanky: 22 June 2012 - 11:00