More police should be recruited to deal with the severe threat from terrorism and potential public disorder, the Police Federation has said.
Around 5,500 officers have retired since 1998 and more than 1,000 are expected to leave within four years, chairman Terry Spence added.
Each year around 3,000 are assaulted while carrying out their duties. There is a freeze on recruitment.
Mr Spence told the Federation annual conference: "Despite the headline costs, extended periods of overtime is the cheaper alternative to bringing the PSNI up to effective operational strength.
"But now that it looks clear the current severe threat of terrorism and the potential public order volatility will endure, then a more realistic and honest answer would be to start recruiting again.
"It is unacceptable that we should have to wait until further atrocity to make the case to the Executive (of ministers at Stormont) for more officers."
Mr Spence said officers were under unacceptable physical pressure - some working 12-hour shifts - leaving them fatigued and burnt-out in a job where they are expected to be alert and energetic.
He added: "The official cliche is that we need to do more with less - an expression which is convenient, glib nonsense. We cannot deliver the proper level of service without adequate numbers of officers."
Justice Minister David Ford recalled in March that chief constable Matt Baggott said he was satisfied with the PSNI's operational capability within its 2010 budget allocation. An extra £245 million funding was secured to deal with security issues.
Mr Ford said: "The chief constable has also assured me that he will keep officer numbers under review and if a compelling requirement exists to increase numbers that he will make a case to my department in such circumstances. I hope, Mr Chairman, that my department has shown that we do listen when cases are made to us."