An Xstrata spokesperson told ILN the longwall was running but not yet up to full production.
Preparations to return to production were underway before Easter, while removing all the water from the underground workings proved to be challenging over past months.
The resumption of mining at Ulan follows a ramped up recruitment campaign to find 350 skilled workers for the planned $880 million Ulan West longwall operation.
The project received New South Wales government approval in November, with the larger Ulan mining complex allowed to double production from its previous limit of up to 10 million tonnes per annum.
Under the approval, the resumption of open cut mining at Ulan cannot extract more than 4.1Mtpa of raw coal, while the whole complex cannot export more than 20Mt in a calendar year.
The Ulan mine produced 4.79Mt run of mine in 2010, averaging just under 400,000t per month.
