But while Labour has been on the attack over Mr Key's oversight of the GCSB, Mr Shearer said yesterday that the party might consider backing a change to the legislation.
"If that comes out as the result of an independent review, and it's carefully prescribed and recommended on the basis of the balance of intelligence versus privacy, then we would certainly look at it," he told the Herald.
However, any changes needed to start with a full independent inquiry into New Zealand's intelligence agencies, something the Government has so far resisted.
"An inquiry's necessary because it's not just about the GCSB, it's also about the SIS and the relationship with the police. It's right across the board and what I don't want to do is to have a narrow, quick-fix Bandaid type of solution for something that is much more fundamental."
Mr Shearer said Australia had independently reviewed its intelligence agencies twice in the past 10 years and New Zealand last did so in 1976.
"We should be following their lead and looking into all these issues so that people can have confidence."
Meanwhile, after Labour's attacks on Mr Key last week for choosing not to halt GCSB operations in support of the SIS when he first learned of potential problems last July, the Prime Minister yesterday emphasised the co-operation between the agencies was nothing new when he came to office in 2008.
"GCSB prior to the 2003 act were always doing this sort of activity," he told TVNZ's Q+A programme.
"It may have increased a bit more over time, but they were doing it. Helen Clark [PM at the time] challenged when the law came in, 'Are you sure we can actually do this?' And the advice she got was, 'Yes, you can, and it's consistent with what we see operating around the world."
Shearer warns Mighty River investors
David Shearer On the eve of the Mighty River Power share offer opening this morning, Labour warned potential investors it will shake up the electricity market when it is next in government.
Finance Minister Bill English and State Owned Enterprises Minister Tony Ryall gave the final tick to the share offer yesterday.
Ordinary investors have three weeks to apply to buy shares, starting today.
The ministers expected the company would be listed on the sharemarket on May 10.
But Opposition Leader David Shearer yesterday warned of Labour's intention "to make changes to the electricity sector to stem the relentless rise in power bills and to ease future pressure on prices from foreign investors looking to boost profits at the expense of consumers".
He said New Zealanders considering purchasing shares in Mighty River Power should be aware of Labour's plans before making the investment.
"The Mighty River Power offer document already includes regulatory risk," Mr Shearer said.
"But while we will detail our proposed changes in the near future, we believe it is responsible to signal our intentions now."
