The phrase "a task that was" is correct and usable in written English.
It can be used when referring to a specific task that has already occurred or been completed in the past.
Example: "The project was challenging, but it was a task that was necessary for our team's growth."
Alternatives: "a task that had been" or "a task that is completed".
Mayor Michael Bloomberg seems committed to watershed protection, a task that was nowhere near the top of his predecessor's agenda.
In theory, the battalion's specialty was guarding enemy prisoners of war, a task that was expected to be a major logistical problem.
The advisers themselves felt they were competent professionals performing a task that was difficult but not impossible, and their superiors agreed.
The postponement also made it necessary to remove and recharge the six batteries in the Hubble spacecraft, a task that was completed on Saturday.
In a task that was not particularly easy, Zillo's staff sifted through these requests to figure out whom the players will visit.
The union said officers were not well-equipped to handle corpses and were not paid extra to perform a task that was not really police work.
To insure that Betty wasn't somehow acting on instinct, Kacelnik and his colleagues "gave her a task that was the opposite.
That is a task that was pioneered by Auto-Tune software, a plug-in for audio software used in recording studios.
Though his pattern-detection algorithms could not distinguish exactly which objects the volunteers had seen, they managed a task that was only slightly less demanding.
The plot quickens as Wikus, who works in alien relations, is told to oversee the mass deportation of the prawns to a new camp a task that was made easier for the filmmakers when they found a shantytown, built on landfill near Soweto, from which real South Africans were about to be moved.
Second, TAs in RPD1 were developing revised course materials for a learner-centered lab experience a task that was new for nearly every TA teaching that semester.
