Mark Lundy's defence team has questioned a forensic scientist over the methods she used to ascertain there was probably brain tissue found in a stain from Lundy's shirt.

Dutch forensic expert Dr Laetitia Sijen remains on the witness stand in Lundy's retrial for the murders of his wife and daughter.

She's been questioned by defence lawyer David Hislop about the tests she ran, as the control sample was different to the test samples used.

He asked her if that was like comparing apples with lemons.

"I don't think he'll start comparing apples with lemons," she said.

"Maybe you would take a bigger lemon and smaller lemon."

Hislop has also put to her concerns raised by an expert called upon by the defence team to assist with scientific evidence.

He says the expert has raised four concerns with the way Ms Sijen tested matter found on Mark Lundy's shirt - to reach the conclusion it probably contained brain matter.

But Ms Sijen says the system she set up to test the matter addressed such concerns.