John Key's son Max has posted his support for his father on social media after the Prime Minister announced his decision to resign today.

Max said he was proud of his father, describing him as the "the best role model I could have asked for".

He said John Key had achieved a childhood dream and "showed me that dreams can really be achieved".

The Prime Minister cited family reasons for stepping down, saying the job had required great sacrifices "from those who are dearest to me".

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In his Instagram post, Max Key said: "Politics is an inherently polarising industry and therefore the journey was not always the smoothest of sailing. In saying this, you always kept your head held high and always remained infectiously positive . . . Love you dad."