ADELAIDE, March 9 - The Shadow Minister for Early Childhood Education and Development and Shadow Minister for TAFE and Vocational Education said that while she loved her job and loved representing the people of her Adelaide electorate, ultimately the decision to quit was for "one 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 in a letter to voters.

"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."

But Ellis, 39, has no plans to force a by-election in Adelaide, which she holds with a margin of about 4.6 per cent, although she will step down as Labor's spokeswoman for Early Childhood Education and Development.

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

He has described Ellis as a "trusted adviser and a true friend".

"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."

Federal Education Minister Simon Birmingham also wished the Labor frontbencher all the best.

"We don't always agree but I respect your commitment and advocacy ... and your love of family," Senator Birmingham said in a tweet.

South Australian Greens Senator Sarah Hanson-Young said her party would target the seat of Adelaide at the next election.

"The race is on in Adelaide and we will be pulling out all the stops to turn Adelaide Green at the next election," she said.

"The people of Adelaide deserve a strong progressive voice in Canberra. They need someone who will stand up for job security, housing affordability, equality and the environment."

There's sure to be strong interest in Labor circles to replace Ellis, with lawyer Jo Chapley, who made a bid for the South Australian parliament in 2014, among possible candidates.

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

AAP