A319 First Officer reported the Captain refused the aircraft due to a Hydraulic Leak. Gate Agent repeatedly tried to board the passengers after being told by Maintenance the aircraft was out of service.

Narrative

On [DATE]; I was assigned to fly a leg from ZZZ to XXX. When we received the initial flight package through the app we had originally been assigned a different aircraft. However; prior to arriving at the gate; we had received a change indicating that we would now be operating a different aircraft. In looking through the new flight package; it was clear that several maintenance items were being addressed. One was for a low hydraulic fluid indication. However; what had not been communicated to either me or the Captain was that the aircraft was out of service with a major unidentified leak and a previous flight crew had been removed from (or rejected) the aircraft. Based on the information that was being communicated through the usual pre-flight information; both the Captain and I had been under the impression that any Maintenance that was being addressed was minor and would not affect dispatch of the aircraft. In addition; we were not made aware of any abnormalities upon checking in with the Gate Agent. I was first to the aircraft and did an interior cockpit inspection and confirmed that the hydraulic level was low for the yellow system. It was indicating about 60-50% of its normal range. I then proceeded to go talk with Maintenance to get an update on work being performed and; in the process; complete an exterior inspection. When I reached the maintenance personnel; there were there folks in the forward cargo compartment doing work with several cargo panels removed. I then spoke with the Maintenance Lead who informed me they had not identified the leak; it was a major leak; the aircraft was out of service; not available for dispatch; and it would be 'a while' before they would be able to determine the cause of the leak. He was extremely surprised they were trying to dispatch the aircraft as he had taken it out of service given the issue. I then returned to the cockpit and informed the Captain. He then proceeded to go tell the Gate Agent not to board. Upon the Captain returning to the cockpit; the Maintenance Lead says they identified the problem and the entire hydraulic line will have to be replaced and it will take two full work shifts to fix the issue overnight. He says also he has again communicated the aircraft is out of service and will not be flying until the next day. Simultaneously; the Gate Agent has communicated with the Purser they are ready to board. The Captain then immediately returned to the Gate Agent to again tell them not to board and relay all information the Maintenance Lead had given. However; the agent and operations delay the flight by 2 hours even though the aircraft isn't actually going to fly and is not airworthy. During this time; Maintenance returns to the cargo hold to resume work. About 5 minutes after they returned; a catering truck then pulls up to the aircraft even though the aircraft is out of service. The was a dangerous turn of events as the truck approached the aircraft and the maintenance crew was unaware it would be arriving in the immediate vicinity of where they were doing work and had work equipment (e.g. ladders) on the ramp. The catering truck could have struck an unsuspecting maintenance crew member as they were performing work; and they would possibly have been unable to hear it approaching because of the hearing protection being worn. At this point the Captain had also rejected the aircraft and Maintenance had told catering the aircraft isn't flying and they need the space for their work. The Captain and I now deplane and he is communicating directly with the passengers and being direct and open about what is going on. The passengers are frustrated (some are angry) because they have been given a very different message initially by the Gate Agent and operations that the plane is only delayed for maintenance. The Captain informs them that we are now working on getting another aircraft and he does not have an estimate on when one will be available. We depart the gate area to communicate with Dispatch and reassess the situation. During this time; the flight was canceled and we were released from duty. This unfortunately is a situation where on-time dispatch took priority over safety.