Northern NSW rivers have become shark creches as dozens of pregnant females head in to the waterways to give birth.

And although the Manning River has hosted bull sharks giving birth to litters of between one and 10 pups for as long as anyone can remember during the summer, the size of one recent "catch and tag" has veteran fishermen shaking their heads.

Two weeks ago, Adam Maddalena and his mate Glen Lambert were catching and tagging sharks for the NSW Department of Primary Industries when he landed a monster from the Manning, up near Taree.

It's not uncommon for bull sharks to swim up river as far as Wingham — about 40km from the coast — but the size of the beast had the veteran fisherman agape.

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He'd hooked a 3m pregnant bull, the biggest he's ever caught in the Manning River.

"We've got our baits out for anything that comes along but we only target the smaller ones so we can tag them and send them off," Maddalena, 38, said.

The duo dropped their baits around 4.30pm. About an hour later Maddalena's line took off.

"We actually thought it was a little one because it was only little bursts of speed so I took up tension and hooked it. I knew straight away it was a lot bigger than we expected," he said.

"We were actually speechless. We sort of just looked at each other and realised how big it was so there wasn't too much said, we just knew that we had to get in and get it done," he said.

"That big one just happened to come in at the time we had our baits out. It was a bit of a shock actually.

"It took me about 45 minutes to get this shark in close enough so we could grab the tail and get a tail rope on it."

Working together, Maddalena and Lambert managed to free the hook and tag the shark before releasing it back into the river.

"Even the head of it was a lot wider than standard so she was a pedigree, pretty much. She was a very healthy shark," he said.

"We made sure we kept her in deep water so there was no pressure on her. She was obviously in pup," Maddalena said.

A NSW Department of Primary Industries spokeswoman said many bull sharks had been caught and tagged in the Manning River over the years.

Maddalena and Lambert average about 30 catch, tag and releases per season.

"All rivers in northern NSW have females visiting to drop their pups at this time of year," the spokeswoman said.

The department is now tracking 42 bull sharks, 12 tiger sharks, and 247 white sharks, she said.