National MP Denise Lee's emotional Facebook post on her son's death

National MP Denise Lee has written a heartbreaking tribute to her son who died at the age of two.

Lee, the MP for the Maungakiekie electorate who entered Parliament following the 2017 election, posted on Facebook on Friday on what would have been her son Riley's 20th birthday.

"[Twenty] years ago today, my son Riley came into the world," she wrote.

"He lived for two beautiful, fun-filled years before unexplainably dying in his sleep.

"Nothing can fill the gap when we are away from those we love, but remembering the good times and the support I’ve had surrounding me over the years really helps."

Lee included six photos of Riley in the post, before signing it off with: "Happy Birthday Riley".

There was an overwhelming response to the Facebook post, with many sending their love and thoughts.

"He's so beautiful and I can't imagine the pain you have experienced. My thoughts are with you," said one Facebook user. "He was so beautiful. What a treasure to have had albeit for such a short time. Loving thoughts surrounding you all today," said another person.

Lee told Newshub on Friday that she had come to "peace with the emptiness" and that it was a "reminder that he both lived and died, and I should now focus on what counts in my life, making a difference if I possibly can".

"The response was completely unexpected and quite overwhelming. It's very special that people are celebrating his life.

"The biggest fear for someone who has lost a child is that they will be forgotten. Today reminds me he isn't."

Friday's Facebook post is not the first time she has spoken about her son. In her maiden speech, the former Auckland Council city councillor said the tragedy was followed by a "series of random interactions with a cold-hearted function-driven system".

"The failure of police inquest officers, pathologists and coroners to sensitively inform and communicate their process to two shell-shocked parents still mystifies me today."

Lee said at the time that difficulties with the coronial process, and its improvements since, led her to understand the impact of the role of an MP.

"I'm subsequently relieved the coronial system has improved for people," she said. "The 2006 Coroners Act and later reviews better protect the interest of grieving families.

"Politics really did become personal for me then. A flick of the pen, wording of an amendment, an exchange in the debating chamber - parliament's processes affect everyday lives."

Lee is the National Party spokesperson for Local Government.