"I think the Maori seats are up for grabs and we are going for broke to get them. We are in competition with the Maori Party and Mana and we are determined to win the Maori seats back."
Mrs Turia, who is a Ratana morehu (follower), said Mr Shearer was a poser for turning up for the church service - a day which should be politics-free.
"I think it's false, because his reasons for coming are not to honour the manaakitanga - his reasons for coming are to try and influence the morehu [followers] vote. It's a laugh."
She also rejected his claim that the Maori electorates were ripe for the taking or that the Maori Party was vulnerable.
"He's got to stop posing and realise that Maori people no longer sit at home and believe all the garbage that comes out of Labour's mouth.
"If they blithely think that's just because we have a leadership change that the Maori Party won't continue to function in Parliament the way it always has, they are quite wrong."
Mrs Turia also called for the alliance between Labour and Ratana to end, saying it gave undue influence to one group of Maori to select a candidate for Labour.
She said nobody could tell the Maori people how to vote any longer and she believed the Ratana Church should no longer be affiliated to any one party and should instead use its influence to improve the lot of Maori.
In previous years, Mrs Turia has criticised politicians as "vultures" for travelling to Ratana to seek votes.
She said she still felt that way. Although she attended in her capacity as morehu, she did not speak, and other Maori Party MPs also did not speak on the marae.
Mr Shearer is due to deliver his first big speech of the year tomorrow.
While it is not likely to contain a policy announcement, Mr Shearer said he would explain further Labour's focus on "hands-on" Government.
Mr Shearer hopes that the different approach he is taking will distinguish Labour from National and be enough to grab attention and set Labour up for 2014 after four years of effectively being ignored by voters.
"By 2014 we want Labour to be in a position where people think 'this is an alternative government, we like what they are saying'."
Its plan to build 100,000 affordable homes was the first policy released under that theme, and he said he would also focus on jobs.
Mr Shearer said he did not believe a more interventionist approach would put people off.
Labour's likely coalition partner the Green Party has timed its annual State of the Planet address for the same time as Mr Shearer's.
Mr Shearer's speech will be at a party summer school in Wainuiomata, while Green co-leader Metiria Turei will deliver the Greens' at a picnic in Mt Eden.