Prime Minister John Key is not ruling out Maggie Barry becoming a National list MP after the gardening guru missed out on winning the party's candidacy for the safe seat of Botany.

Ms Barry, who put her name forward for the Botany byelection after resigning her RadioLive hosting role late last year, was beaten to the nomination by Auckland councillor Jami-Lee Ross.

Mr Key said Mr Ross had great credentials - he was elected twice to the Manukau City Council before winning a seat on the new Auckland Council, and has been a party member since 2003.

"But I also think Maggie Barry has a contribution that she can potentially make and in due course we will have some discussions with her."

Asked if Ms Barry would become a list MP, Mr Key would not rule it out.

Speaking in Botany yesterday, where he helped to put up Mr Ross' first billboard for the March 5 ballot, the Prime Minister said byelections were always a challenge, even in traditionally safe seats.

The challenge in Botany would be to energise people to get out and vote, given that a general election would take place later in the year, he said.

"It may reduce the turnout here in Botany but we have a tremendous candidate in Jami-Lee Ross. He has 100 per cent of my support."

Mr Key did not see the byelection as a litmus test for Government plans to partially sell up to $10 billion worth of state assets, saying byelections were often skewed by a lower voter turnout than general elections.

The Labour Party has chosen Michael Wood, a member of the Puketapapa Local Board and former Pakuranga candidate, to represent it in the byelection.