Liam James Reid told a Christchurch High Court jury that scientific evidence against him in the case of 20-year-old Emma Agnew must have arisen from accidental transference, contamination, or from being planted, the Christchurch Court News website reported.
In the case of the 21-year old Dunedin student he told crown prosecutor Pip Currie: "Your investigation has so many holes you could strain a cup of tea through it".
Reid has pleaded not guilty to raping and murdering Miss Agnew in Christchurch in November last year and raping and attempting to murder a student in Dunedin nine days later.
Reid suggested he must have walked into the blood of the 21-year-old Dunedin rape victim later, after she had left blood spots on a footpath.
He believed it must have then been transferred to the top of the shoe and the blood on the sole had disappeared from walking around the city.
He referred to "the dodgiest ESR investigation I have ever heard of" when Mrs Currie asked him about his DNA being found on the rope used to choke the Dunedin rape victim.
He said the scientist had said the DNA could have come from two other sources and was not a perfect match for him.
Asked about his DNA being found under the rape victim's fingernails, he said it was "inconceivable".
He said police were trying to place him in the investigation and were stretching tiny bits of information.
He said witnesses were just saying what the police wanted them to say, or were mistaken.
He described the DNA evidence as "a botched investigation".
The trial resumes with one last defence witness tomorrow and then closing addresses by the crown and defence will begin.
Justice Lester Chisholm is likely to sum up for the jury on Thursday.
