One outgoing New Zealand First MP says being with National would be a very powerful position for Winston Peters.

Richard Prosser told Mike Hosking he thinks Mr Peters will swing left, but hasn't ruled out sitting on the cross benches.

However he does think the days of formal coalitions where members from all parties sit inside cabinet are probably over.

"And if you are the smaller party within a larger cabinet you are pretty much tied to the larger party's policy agenda and don't really have any ability to rebel against that without breaking things up."

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