The Mid Ulster District Council construction workers found further problems in the swimming pool substructures, as well as additional plumbing and electrical requirements.
These works will see the reopening of the centre pushed back to January 2020.
Councillor Trevor Wilson, Chair of the Council's Development Committee, said: "In a building as old as Dungannon Leisure Centre which dates from the early 1970s, there is always a risk that, when you begin to strip it back, you will uncover problems that hadn't previously been anticipated.
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"While this moves our re-opening from the autumn of this year to the early part of 2020, the important issue is that we take the time needed to ensure the building is fit for purpose and ready to offer high quality health and leisure activities to the people of Dungannon and the surrounding area.
"We know that many of our customers have been waiting patiently for the centre to open again and we apologise for asking for their continued patience while we manage this additional work."
The newly refurbished leisure space will feature upgrades to the swimming pools, fitness suite, changing facilities and reception area. Renovations to the pool include new tiling, pool linings, surrounds, filters and structural work, while the gym will be fitted out with a completely new range of fitness equipment and the centre's squash court will also be revamped.
(CM/JG)
