Francois Moller says he has been able to access the deleted messages from Mr Pistorius' mobile phone.
Mr Pistorius has said he has forgotten the password to his phone.
He denies deliberately killing his girlfriend Reeva Steenkamp, saying he shot her by mistake, thinking she was an intruder.
The trial has now entered is fourth week, with the prosecution expected to wrap up its case before Friday.
Capt Moller said he had been able to extract some 35,000 pages' worth of messages from two iPhones and two Blackberrys he had been given from Mr Pistorius' house.
Earlier, one of Mr Pistorius' neighbours said she had heard both a man and woman scream on Valentine's Day 2013.
Anette Stipp, whose husband gave evidence earlier in the trial, said she heard the screams and gunshots at around 0300 local time on the day of the shooting.
She said she heard two groups of gunshots with a woman screaming in between.
Her testimony has closely matched that of other neighbours and witnesses, including her husband.
The defence has previously claimed that Mr Pistorius screams like a woman and that neighbours had confused his screams with those of Ms Steenkamp.
"It was a definitely a woman screaming," said Mrs Stipp.
She also said that the light was on in the bathroom cubicle, where Ms Steenkamp was when she was shot.
This contradicts evidence given by Mr Pistorius, who said that it was "pitch dark" where the shooting took place.
The BBC's Pumza Filani in the court in Pretoria says the witness remained resolute in the face of intense cross-examination.
On Sunday the trial was extended and will now run until the middle of May. It had been due to end this week.
The prosecution says it will call upon four more witnesses before closing its case.
Oscar Pistorius says he shot his girlfriend Reeva Steenkamp by mistake after mistaking her for an intruder
The defence team, led by Barry Roux, will then call upon its own witnesses, including Mr Pistorius himself.
Last week the trial heard evidence from ballistics experts and computer forensic teams who described the sequence and timing of the shots that killed Ms Steenkamp.
There has been intense media interest in the case in South Africa and beyond because of Mr Pistorius' status as a national sporting hero.
He is a double amputee who holds six Paralympic medals and also competed in the 2012 Olympic Games.
The prosecution accuses Mr Pistorius of intentionally shooting Ms Steenkamp - a model, reality TV celebrity and law graduate - after an argument.
But the athlete maintains he believed his girlfriend was in bed and that an intruder had entered the bathroom when he shot at the toilet door in the early hours of 14 February 2013.
There are no juries at trials in South Africa, and his fate will ultimately be decided by the judge, assisted by two assessors.
If found guilty, the 27-year-old - dubbed the "blade runner" because of the prosthetic limbs he wore to race - could face life imprisonment.
1. Balcony
Mr Pistorius said in his statement at the start of the trial that he woke in the early hours and walked on his stumps to the balcony, pulled in two fans, closed the sliding door and drew curtains. He said that shortly before he had spoken to Reeva, who was in bed beside him.
He said he rejected prosecution claims that a witness heard arguing coming from the house before the shooting.
2. Bathroom noise
Mr Pistorius said he heard the bathroom window sliding open and believed that an intruder, or intruders, had entered the bathroom through a window which was not fitted with burglar bars.
"Unbeknown to me, Reeva must have gone to the toilet in the bathroom at the time I brought in the fans," he said.
Mr Pistorius said he approached the bathroom armed with his firearm, to defend himself and his girlfriend, believing Ms Steenkamp was still in bed.
3. Shooting
Both sides agree four bullets were fired. Ms Steenkamp was hit three times.
Mr Pistorius said he fired his weapon after hearing a noise in the toilet which he thought was the intruder coming out of the toilet to attack him and Ms Steenkamp.
He said he was in a fearful state, knowing he was on his stumps and unable to run away or properly defend himself.
Mr Pistorius said he rejected claims that he was on his prostheses when he shot at the door.
A witness told the trial she woke to hear a woman screaming and a man shouting for help. She said that after the screams she heard four shots.
4. Bedroom
At his bail hearing last year, Mr Pistorius said he went back to the bedroom after shooting at the toilet door, then noticed Ms Steenkamp was not in bed.
Mr Pistorius said he then realised she could have been in the toilet.
5. Toilet door
Mr Pistorius said he went back to the bathroom but the toilet was locked, so he returned to the bedroom, pulled on his prosthetic legs, turned on the lights before bashing in the toilet door with a cricket bat.
Forensics expert Johannes Vermeulen told the court that the height of the marks on the door caused by the cricket bat suggest Mr Pistorius was on his stumps at the time.
6. Emergency calls
Mr Pistorius's defence team has said he then called security at the gated housing complex and a private paramedic service before carrying Ms Steenkamp downstairs.
But security guard Pieter Baba told the trial he had called Mr Pistorius first, in response to neighbours' reports of gunfire, and not the other way round.
He said Mr Pistorius had told him: "Everything is fine," before calling him back a few minutes later and crying down the phone.
Source: BBC News
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