The three-day event was scheduled to start on Friday.

Field days chairman and college principal Cameron Archer made the announcement on Wednesday night.

"We regret having to cancel the field days, but it is the only responsible thing to do," Dr Archer said.

Dr Archer said the key problem - apart from more heavy rain forecast - was a severely damaged concrete bridge at Mindaribba, on Tocal Road, which was the main access road to the event.

"This bridge was damaged by the recent storm event and is unstable," he said.

"With further rain and heavy traffic there is a possibility that it could fail completely."

Dr Archer said council had assessed the risk for slow, single-lane traffic, but the question was how it would hold up with the heightened traffic expected for the Tocal Field Days.

The annual event attracts up to 10,000 visitors a day.

"Council and emergency services were concerned about how it would hold up," he said.

"Only one lane is being used."

This would leave limited access to the Tocal campus through secondary country roads in areas that were subject to flooding, he said.

Heavy rain could cause the Paterson River to quickly rise and make Gresford Road to the north and Paterson Road to the east unpassable.

Dr Archer said the event involved 12 months of planning.

"We and many others plan all year for the event. We look forward to it and enjoy all aspects of it," he said.

Click here for a gallery of photographs from past Tocal Field Days events

"This year, all sites were fully booked up and ready to go. [The cancellation] is a bit sad but we will get through it. We've always known it could happen -  it was just a matter of time."

Many exhibitors had already set up displays and had been looking forward to the weekend, albeit a wet one.

Dr Archer said previous field days had been affected by rain but this time the landscape was already sodden from last week's super storm and subsequent flooding, and the Paterson River remained above normal levels.

"The Tocal Field Days Association and the college work assiduously on safety; we take great care and we take our responsibilities seriously," Dr Archer said.

"Sometimes hard decisions have to be made in the interests of eliminating risks completely.

"We do not believe we can take the risk of running the event given the advice we have had as well as the access problem which is evident when one assesses the condition of the bridge at Mindaribba," he said.

Dr Archer vowed that once the site was packed up, the team would work towards the 2016 field days with "renewed vigour".

"We have many committed and supportive exhibitors and community groups who work so hard alongside us and we look forward to working with them for the 2016 field days," he said.

He paid tribute to the efforts and contribution of field days manager, Wendy Franklin, and event services officer, Carol Cairney, who he said worked all year for these three days.

"They, along with all at Tocal are disappointed that the event has had to be cancelled," he said.

Dr Archer said he had been agonising over the decision for two days but a photograph of the bridge, sent to himWednesday afternoon by Maitland City Council, cemented his decision.

"It had a huge big crack," Dr Archer said.

With Maitland City Council engineers, SES personnel as well as police and ambulance advisors all urging cancellation, Dr Archer called it off.

"I was loathe to against the advice."

The east coast low weather system expected later this week could also bring further complications, Dr Archer said.

"If we get more rain in the catchment and the bridge becomes more unstable, these people might not be able to get out."

Dr Archer said Tocal was yet to assess the financial impact of the cancellation and said postponement to a later date in 2015 was not an option.