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The New Zealand First leader is well-known for his refusal to name parties with whom he will be prepared to deal after any election in which he may be the kingmaker.
In the first MMP Parliament, he kept New Zealanders waiting for weeks while he made up his mind who to support.
The 2014 election is shaping up to provide Mr Peters with another opportunity to influence the formation of the next government. If National slips further, and if Labour and the Left continue to gain ground, Mr Peters comes into play.
But already the NZ First leader is setting some rules. He wants a royal commission of inquiry into allegations the former justice minister was involved in a campaign to undermine former Serious Fraud Office head Adam Feeley.
''Our position, and it is rock-solid from which we will not deviate, is we want a full-scale commission of inquiry with a respectable commission that we can trust and with terms of reference that we can trust,'' he told Radio New Zealand.
Mr Key ordered the inquiry after an email emerged targeting Mr Feeley while the SFO investigated collapsed investment company Hanover Finance in 2011.
The email, sent by Whale Oil blogger Cameron Slater, said Ms Collins was ''gunning'' for Mr Feeley.
She resigned on Saturday, but vehemently denied the accusations.
Mr Peters said the inquiry should not have been instigated before the election because the Government was effectively in a caretaker role.
Mr Key said the inquiry would be carried out by a retired judge or a QC, who would have full powers under the Inquiries Act to conduct an independent investigation into the allegations, and would report to the Prime Minister.
He expects the inquiry will take weeks to complete its work, and is likely to be reporting back after the election.
Labour and the Green Party have also criticised the narrow terms of the inquiry. However, none has the power to sway the inquiry quite like Mr Peters, who is watching NZ First comfortably polling above 5% in the latest opinion polls - ensuring his return to Parliament.