Project manager Adrian Cameron struck a convict-built storm water drain while in the process of upgrading the reticulated water main between Hill Street and Darling Street.

It's no small discovery, with the tunnel at least 15 metres deep nearly two metres underground, in the top of each brick is the thumb mark of the man who made it about 100 years ago.

"I think back in the day when the roads were first surveyed, there would have been a lot of gullies and things crossed with old logs, you didn't want your horse and sulky going up and down too quickly," Mr Cameron said.

"I'm thinking over time they have put these brick archways under the road to make it flat and eventually they became part of the storm water network.

"It looks like when they have done the job a lot of pride was taken, even down in the hole the brickwork is as nice as the outside of the house."

Anyone who's driven down Carthage Street between Hill and White Streets would have unknowingly crossed the piece of history, and the workers think there could be more quirky discoveries along the way.

The bricks themselves appear to be formed by hand and the tunnel has stood the test of time.

There's no detailed plans on what lies beneath the roads from that period, Mr Cameron said, but it's not unusual for construction workers to find relics of the past in East Tamworth.

"Some of the other guys have found old gutters and brickwork in Fitzroy Street which are stamped like cards with diamonds; clubs; hearts; spades," he said.

"Apparently if you get a full set of those they are worth quite a bit with the full deck of cards.

"They used to call them convict bricks, when you had a lot of available labour, it would have been fairly forced but the guys used to form up the bricks and lay them which is a really big job."

The tunnel looks like it was once a road crossing and was later made to be part of the city's storm water network until it was removed from service.

Heritage experts will take a look at the tunnel but there is no plan to demolish it; at least 60 cubic metres of concrete sand has been ordered to fill the hole to make the road above safe.

The construction workers might even leave something in there for the next person 100 years from now, Mr Cameron said.

"The guys were hoping for the 1938 penny, we didn't find any but we might have to leave something from 2020 for when it's next dug up," he said.

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"We think there will be other things, there's not a lot of good history or plans from that period so it's just a matter of as we're digging along being careful to see what we find.

"I'm really surprised, the workmanship that was done, the seal, it just shows the pride in their work and the effort that went in because it's basically as good as new. They made it good in the old days."

Mr Cameron estimated it would have taken the convicts about a month to build the tunnel and make the bricks by hand, but without a union there probably weren't a lot of lunch breaks.

The find has slowed the works on Carthage Street for the next few days until the tunnel is deemed safe and filled with concrete sand.