Fisheries officers are believed to have narrowly missed catching a large bull shark just hours before it attacked a surfer at Ballina in northern New South Wales on Tuesday evening.

The revelation came as an expert said the latest attack was not unusual given the species involved and the location.

Champion surfer Sam Morgan sustained a serious bite wound to his left thigh after being attacked by the suspected bull shark at Lighthouse Beach about 6.25pm.

After examining bite wounds, government experts suspect the animal was a bull shark - a species notorious for attacks in murky shallow water near river mouths, especially after heavy rainfalls in the catchment.

The State Government has mounted a series of shark attack prevention measures including a tagging and tracking program in northern NSW waters.

Earlier on Tuesday a helicopter patrol working for the government-funded shark tagging program spotted a large bull shark in the mouth of the river near Lighthouse Beach, according to Ballina Shire Council mayor David Wright.

Cr Wright said the helicopter immediately alerted a shark-catching boat that was just off the river mouth.

But he said they were unable to catch the animal as it refused to take the bait.

"It was near the Missingham Bridge and they couldn't catch it. It could easily have been the same shark," he said.

The attack takes the number of shark incidents to 12 this year on this stretch of coastline.

But the latest incident was "no freak event" with water conditions in the area and the timing being favourable for bull shark activity, according to Southern Cross university senior lecturer in marine biology Daniel Bucher.

"Water levels were quite up and we had heavy rain with the river flowing (out to sea)," he said.

"And we know coming into late spring early summer that it is breeding season with sharks coming up into estuaries to pup. So the bigger ones are coming in at this time of year.

"I wouldn't call it a freak event. If you have lots of people hanging around river mouths surfing someone is going to bump into a bull shark, especially [when] the rain has added to the risk."

Mr Bucher said bull sharks' behaviour differed from great white sharks.

"They are a scavenger and if they see a large animal that doesn't look healthy then they will bite and see if it puts up a fight," he said.

"Unfortunately a lot of deaths from bull sharks occur ... from bites on the leg where they sever an artery and the person loses too much blood and goes into cardiac arrest or drowns.

"It's not that they [bull sharks] bite people in half, they are not that kind of shark."

Mr Bucher said there had also been other bull shark attacks in and around the Lighthouse Beach area in recent times.

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"They are seen there regularly, it's not uncommon."

He also cautioned that the current government-run program of aerial patrols to address attacks might have more difficulty dealing with bull shark risks than great whites.

"They [bull sharks] do tend to hang around dirty water. Aerial surveys are not going to effective because they can't see into dirty water, and they are not doing flights at dawn or dusk and can't see into the water when the sun is low in the sky.

"So in many ways they are a more difficult shark to deal with because the also come into those shallow, sheltered waters".

This weekend Ballina is expected to see an influx of tourists for the town's annual Prawn Festival which involves river activities.

Cr Wright said extra aerial and jetski patrols would be operating to ensure festivities scheduled to take place in the river went ahead safely.

He also said the State Primary Industries Minister, Niall Blair, had contacted him today promising a long-term program of measures to reduce the risk of shark attack.

Cr Wright said the five-year program included fast tracking the eco-netting of Lighthouse Beach and extra aerial patrols.

He said if the netting worked at Lighthouse Beach then it would likely be rolled out at other beaches as well.

Cr Wright said another measure for which he was hoping the State Government would assist with funding included the installation of beach sirens or alarms, which could be activated in response to shark sightings.