Goalkeeper Jerzy Dudek, back in the side with Chris Kirkland injured, looked at fault for the strike which rescued a Premiership point for Pompey.
He could only palm a cross from Matthew Taylor into the path of LuaLua, who gratefully headed home.
Liverpool were shell-shocked and little wonder because they had run a game in which Steven Gerrard gave them a 70th-minute lead. Gerrard had almost opened the scoring in the eighth minute when his shot was parried by goalkeeper Jamie Ashdown.
Then 60 seconds later Dietmar Hamann failed to find the target from close range after Antonio Nunez had flicked on a corner from Gerrard.
They came under pressure again in the 18th minute when Nunez whipped in a good cross to the near post. But defender Linvoy Primus was perfectly positioned and steered the ball to safety.
Portsmouth almost made the breakthrough in the 25th minute after a swift counter-attack.
LuaLua tried to curl a shot from inside the area beyond Dudek, but Steve Finnan got a vital touch.
Liverpool surged forward a minute later, and Gerrard picked out Nunez at the back post. He had time on his side but succeeded only in sending a weak shot into the arms of Ashdown.
Finnan raced forward in the 35th minute and whipped in a cross to the near post. Again, Portsmouth defended staunchly with Andy Griffin booting the ball clear.
The visitors almost paid for a moment of slackness in the 38th minute. Amdy Faye was easily knocked off the ball by Nunez, who broke forward deep into the Portsmouth half. He cleverly released Harry Kewell in a good position on the edge of the area.
The Australian midfielder seemed to be caught in two minds, and his attempted pass to Finnan drifted out of play.
Portsmouth remained dangerous on the break, and LuaLua showed a good turn of pace in the 42nd minute to race into the opposing area. The striker's shot was on target but did not carry enough power to trouble Dudek, who got down quickly to block his effort.
This was a good spell by Portsmouth and a minute later Gary O'Neil saw his well-struck effort go narrowly wide of the post, with the goalkeeper well beaten.
Liverpool regained the momentum at the start of the second half, and Gerrard fired in a powerful shot after 47 minutes which just cleared the bar.
They kept up the pressure, and Kewell made space inside the area only to see his effort go wide of Ashdown's left-hand post.
Then Dejan Stefanovic was forced to make a hurried clearance under pressure from Milan Baros in the 51st minute. Gerrard had begun the move with an incisive pass to Nunez, who delivered a cross from the right.
Portsmouth were again forced on the back foot two minutes later when they scrambled the ball away from Baros after a clever free-kick from Xabi Alonso.
Kewell was again out of luck in the 56th minute when his fierce drive whistled past the post as the Kop held its breath.
Ashdown twice came to his team's rescue in quick succession.
He superbly blocked Nunez's effort in the 58th minute and then somehow managed to parry and then catch Hamann's close-range header.
Kewell then got away from former Liverpool midfielder Patrik Berger but failed to test Ashdown from the edge of the area.
The former Leeds star was finding plenty of room to manoeuvre and saw Ashdown make a good block from his effort in the 69th minute.
Gerrard, typically, opened the scoring in the 70th minute with a 25-yard thunderbolt into the corner following a short free-kick from Hamann.
Portsmouth were looking disheartened, and the home side almost added a second in the 86th minute.
Substitute Florent Sinama-Pogolle saw Arjan de Zeeuw block his shot which bounced into the path of Kewell. He rattled the ball back towards the goal only for Ashdown to get down quickly to smother his effort.
Then came LuaLua's equaliser to leave the Kop in a state of disbelief at seeing two precious home points slip away.
LIVERPOOL: Dudek, Finnan, Hyypia, Carragher, Traore, Nunez (Josemi 89.00), Hamann, Gerrard, Alonso, Kewell, Baros (Sinama Pongolle 82.00).
PORTSMOUTH: Ashdown, Griffin, De Zeeuw, Primus, Stefanovic, LuaLua, Faye (Berkovic 85.00), Stone, O'Neil, Berger, Fuller (Taylor 61).
Referee: M Clattenburg (Co Durham).
Last night's results: FA Cup second round replays: Brentford 2 (Rhodes 52 pen, Talbot 59) Hinckley Utd 1 (Lavery 57). Swansea 2 (Connor 47, Goodfellow 88) Stockport 1 (Griffin 72).