David Seymour's digging in his heels over writing to police, advocating for high profile murder-accused Philip Polkinghorne

Prime Minister Chris Luxon has called the letter ill-advised, but notes Seymour wasn't a Minister at the time.

Seymour says his job as Epsom electorate MP is to be a last line of defence for constituents who think they've been unfairly treated.

He says he wrote a letter detailing a large number of Polkinghorne's concerns -- but says he says he was careful to explicitly say he respects the constitutional separation between police and Parliament.