Union chief Matt McCarten has made a surprise entry into the Mana byelection, saying he intends to put the blowtorch on Labour.

The seat is a Labour stronghold but Mr McCarten believes he can win, saying voters are more likely to take a risk on an outside chance in byelections than general elections.

He said he was not concerned about the chance he could split the left-wing vote in Mana, giving National candidate Hekia Parata the chance to rise through the middle.

Mr McCarten said Labour needed to be pressured over its policies.

While he was "buoyed" by recent indications it was moving away from conventional economic policies, its policy on GST - to keep it at 15 per cent but exempt fruit and vegetables - was "a bit wet and gimmicky".

He said he would stand on three main issues - a $15 minimum wage, employment laws such as the 90-day trial period, and tax reforms that favoured low-income earners.

Labour leader Phil Goff said it was possible Mr McCarten would take some votes away from Labour's Kris Fa'afoi but he still believed it was a contest only between National and Labour. Mr Goff said Mr McCarten had not been a Labour Party member for many years and was now a member or "sympathiser" of either the Green Party or the Maori Party.

His standing in the byelection will also pit union against union - those such as Service and Food Workers and Engineering, Printing and Manufacturing are involved with the Labour Party while Unite members will rally behind Mr McCarten, their leader.

Mr Fa'afoi said he was confident the unionists working on his campaign would stick with him.

Mr McCarten said his decision to stand in Mana was not simply a platform to beat the union drum.

"We've got to try and win. You don't get into a byelection just to try and fly the flag and then go home again."

Diagnosed with cancer this year, he said he was responding well to treatment, which ranged from Western medicine to acupuncture to the "prayers of my kuia and kaumatua".

Although he will officially stand as an independent, it is with the backing of his Unite union.

In 2008, Winnie Laban had a 6155-vote majority in the seat over National's Hekia Parata, now a list MP.

However, the party vote was more even - National polled 2508 votes behind Labour in party votes.

The Greens also polled well, getting about 8 per cent of the vote.