New National MP Denise Lee has spoken of the death of her 2-year-old son Riley as the trigger that got her into Parliament, saying the way they were treated made it feel as if she was on trial for his death.

In a deeply personal maiden speech, Lee, the MP for Maungakiekie, said she had woken one night and gone to check on her son to find he had died in his sleep.

She described the days that followed and interactions with police, coroners and pathologists as "a cold-hearted, function-driven system".

She said the difficulty of getting information and failure to explain what was happening sensitively and with compassion still mystified her today.

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"Trying to keep up with where Riley's body has gone, what they were doing to it, what they were retaining from it, receiving an abruptly worded police letter informing us of our Coroner's court hearing date, it was all too much.

"No explanations, no 'frequently asked questions' brochure, just a summons. You'll understand I thought we were being put on trial for the death of our son."

Lee - the daughter of former National and Christian Democrats MP Graeme Lee - said she had been around politics all her life but that experience was what led her to politics herself.

"Walking through the valley of the shadow of death, trying to understand the legalities and desperately wanting to just stay away from the world to get on with grieving, my sense of indignance grew ... this indignance formed a seed that merged into a big part of the driving force that sees me standing here today."

She said the system had since improved following the 2006 Coroners Act and review of the system so other families did not have to go through the same experience - and that had taught her the different politics could make to people's lives.

Lee also spoke of life as a politician's child, recalling running round the Beehive as a teenager and falling on Robert Muldoon "when he opened his secret, private elevator door".

Lee ended with a message to Riley - "you are a gift" - and for her daughters Sydnee and Makenna.

"Your world is not the one I grew up in. I spent weekends rat shooting at the Paeroa dump, you navigate the virtual world, streaming mass international content 24/7 under the watchful eye of the Google and Facebook empires."

She said that world changed politics and she was proud to have them as guides.

Lee is a former Auckland City councillor and was a former deputy leader of United Future before standing for National in 2014.