can DNA evidence be challenged?

Yes.

DNA evidence can be wrong, overstated or incorrectly interpreted. This evidence can be challenged for admissibility ( for example under section 137 of the evidence act), or at trial.

Laboratory testing may identify biological material and potential contributors, but questions can arise at multiple stages - from how samples were collected and selected, to how DNA profiles were generated, interpreted, and reported.

Challenges may relate to the DNA profile itself. This can include the quality of the sample, the presence of mixed DNA from multiple contributors, the possibility of low-level or partial profiles, and the assumptions used during analysis. You can read an example here of how DNA was challenged in a sexual assault matter.

Statistical interpretation is also a key area of scrutiny. Likelihood ratios, population data, and modelling approaches can significantly influence how strongly a result is presented. Different assumptions or methods may produce different evaluative outcomes.

Handling and contamination are further considerations. DNA may be introduced through transfer, through contact with other items, or during collection, packaging, storage, or laboratory examination. The strategy used to sample exhibits can also affect what DNA is detected — and what is not.

For these reasons, DNA evidence should not be assumed to be correct, complete, or beyond challenge. Errors in profiling, interpretation, statistical evaluation, and handling do occur, and courts recognise this. In some cases, deficiencies are significant enough that applications to exclude the evidence from trial are granted. Careful forensic scrutiny is often required to determine whether the findings are reliable, and whether they should be relied upon at all.