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Dr Clark leaps ahead of his one-time employer and mentor David Parker, who slipped to ninth from second at the last election, when he was deputy leader to the now-departed David Cunliffe.
Dunedin North MP Dr Clark has become one of the party's most effective debaters in the house and, as health spokesman, will be expected to play a central role in Labour's campaign.
Because of his expected nationwide duties, the MP has built a large team of volunteers to campaign on his behalf.
Social media will be used extensively in Dunedin North to motivate voters.
Dunedin South MP Clare Curran, who decided not to take a list placing in 2014, comes in at No22.
Ms Curran was expected to hold her seat, despite her majority falling at the last two elections.
Labour's list ranking committee did Invercargill candidate Liz Craig no favours by lifting her two places to 30 on the list. Dr Craig, a public health doctor, stood in Clutha-Southland in 2014 and is highly thought of in the South for her work within the party and her child poverty advocacy work.
Labour would need to win about 35% of the vote for Dr Craig to be elected - not impossible, although higher than the party has polled recently.
The highest South Island-ranked Labour MP is Wigram's Megan Woods at five. West Coast-Tasman MP Damien O'Connor is at 17 and Port Hills MP Ruth Dyson is at 23.
Clutha-Southland candidate Cherie Chapman is 61 and Waitaki candidate Zelie Allan does not appear on the list.
The release of the list caused controversy when high-profile broadcaster and former Alliance MP Willie Jackson was unhappy with his ranking of 21. However, the party stood firm and Mr Jackson remains at 21, encouraging him to work hard at lifting the party vote to 30% or more to ensure he gets elected.
Because Labour goes for gender equality on its list, Mr Jackson is ranked behind four new women candidates.
Mr Jackson, who defected from the Maori Party in controversial circumstances, has been appointed as Labour's Maori campaign director. Labour's Maori electorate MPs have opted to stand in their electorates only, meaning they need to win them to return to Parliament.
Strangely, it is in Mr Jackson's interest for fewer Labour Maori electorate MPs to win in September, as it will increase his chances of gaining a seat from the list. The Maori Party has reached an agreement with the Mana Party not to stand against each other in the Maori seats, leading to speculation some of the six Labour-held seats are at risk.
Also of note is Napier MP Stuart Nash being ranked at 10 on the list.
Mr Nash is to the right of most of his caucus colleagues and stood for the electorate only in 2014. It seems his rehabilitation back into Labour is complete.
He, Te Tai Tokerau MP Kelvin Davis and former Police Association president Greg O'Connor act as a counter to some of the more extreme views of the Labour caucus.