United Future leader Peter Dunne believes his support in Ohariu has been eroded because of the Jacinda Ardern effect but he questions how long that will last.

The Q + A show has a snap poll tomorrow (TV1 - 9am, Sunday) which is expected to show Dunne trailing Labour candidate Greg O'Connor.

"The question is, and it is something everyone is trying to figure out at the moment, is how deep-seated that factor is," Dunne told the Herald on Sunday.

"Is it a phenomenon that will pass by as quickly as it arose or is it something more substantial?

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"I am not going to get too carried away or excited at this stage because I think there is a lot more water to flow under a rather big bridge."

Dunne has held the west-Wellington seat since 1984, originally as a Labour MP, but held it in the 2014 election by only 710 votes. He has been a support partner of the National-led Government since 2008.

In the past National has campaigned for only the party vote but this time it is explicitly asking National supporters to give Dunne their electorate vote to keep him in Parliament.

Labour and the Greens have done an electorate deal in which the Greens are not standing in order to give O'Connor, the former police union boss, a stronger chance of rolling Dunne. The Greens had 2764 electorate votes last time.

Dunne questioned the "Jacinda effect" in light of Labour's water tax announced this week by Ardern, which he said was "frankly daft".

"I'm in favour of a royalty being charged on fresh water exports, which is quite separate from charging everyone who uses water, which is effectively a rental for it, and which is where their policy has gone way off the ropes," Dunne said.

"The question is 'is that just bad policy development or will we see more of this?'"

He thought it would be more a case of the latter given Arden inability to answer questions about the OCR last week and the Environmental Protection Agency approval for iron sand extraction off the South Taranaki coast.

"It could well be that the 'Jacinda effect' is over before it has started."

Dunne sat with Prime Minister English today at an event to open new premises for a Marist sponsored youth-focused welfare agency in Johnsonville.

English went on to taste a new burger named after him at the Backbencher pub in Wellington.

He then attended a campaign rally in Lower Hutt with Transport Minister Simon Bridges where he announced that a target for one in three cars in the Government fleet to be electric or hybrid electric by 2021.