The "simple truth" behind Kate Ellis' shock decision to quit federal politics.

Senior federal Labor MP Kate Ellis is to quit federal politics at the next election, telling voters she wants to spend more time with her family, particularly her young son.

Ms Ellis has written to voters in her Adelaide electorate telling them the decision to quit was a difficult one, but one made for a simple reason.

“Whilst my son could travel with me as a baby, during the next term of parliament he will start school and needs to stay in Adelaide,” she said.

“The simple truth is that I just cannot bear the thought of spending at least 20 weeks of every year in Canberra away from him and the rest of my family.

“When I think about having to regularly miss things like his first day at school, his presentations at school assembly, a first sporting match or even just being there for him when he is sick and wants his mum, I know that it would make me absolutely miserable.”

Ms Ellis, 39, was elected the member for Adelaide in 2004 and has held the the inner-city marginal seat since then.

She served as a minister in previous Labor governments and is now Labor’s early childhood and development spokeswoman.

Kate Ellis talks to Mamamia on our I Don’t Know How She Does It podcast. Post continues after audio.

Opposition Leader Bill Shorten said he would be sad to see Ms Ellis go.

“We know how hard she works and we admire her strengths as a policy thinker, as a campaigner and as a trailblazer,” he said in a statement.

“But we also know Kate wants the chance to spend precious time with her son Sam. We respect her decision.”

Mr Shorten described the member for Adelaide as a “trusted adviser and a true friend”.

“As a person and a representative, she is someone for whom I have the highest respect,” he said.

“Kate has a rare gift for being at home in any situation. A natural with people, she is just as comfortable reading stories in a primary school classroom as she is talking jobs and apprenticeships on a factory floor.

“Wherever she goes, she radiates an honest charm and a genuine interest in the lives of the Australians she serves.”

Ms Ellis will step down from the shadow ministry. The timing is yet to be determined.

Labor leader Bill Shorten is weighing up whether to undertake a broader reshuffle or make a simple replacement.

Victorian MP Clare O’Neil is considered a strong chance of entering the shadow cabinet.