An industry body predicts one of the two surprise roads to be announced today will require drivers to pay a toll.

Stephen Selwood, Infrastructure New Zealand chief executive, anticipated the roading projects would cost around $800 million and said it was "really positive" that further details would be released today of the unexpected new Whangaparaoa and south Auckland projects.

"I was hopeful - that's the best way of describing it," he said of the surprise news that the new coalition Government backed the deals.

Transport Minister Phil Twyford and Auckland Mayor Phil Goff will today announce details about the new Penlink project between the Whangaparaoa Peninsula and the Northern Motorway and Mill Rd to improve connections in the Manukau-Takanini-Drury areas.

Selwood said: "That's really positive. Penlink will be tolled. It will probably be in the $200m to $300m range as a two-lane which will be woefully inadequate. It should be nearer $400m as four lanes. A PPP is the obvious outcome. It was originally proposed in the mid-1990s by the Rodney District Council."

The Mill Road project was more complicated so was unlikely to be a PPP, Selwood said. Central and local government funding was more likely there, he said.

"This is all really positive but we're still a long way behind what's required when you think of the congestion across the whole roading network," he said.

Mill Rd was likely to be around $600m, Selwood said, although full road widening of the corridor would cost nearer $1 billion.

This month, Associate Transport Minister Shane Jones went on a fact-finding mission to Australia to investigate the best ways investors could work with the Government on public-private partnerships.

Infrastructure NZ this month welcomed the new Government's interest in public-private partnerships.

"The New Zealand Government has restated its committed to resolving congestion and other transport issues in Auckland and across the country. It is widely agreed that conventional funding and financing tools are inadequate to address both the backlog of investment and respond to strong growth," Infrastructure NZ said last week.

"Infrastructure New Zealand welcomes news that the Government is actively investigating alternative procurement options, including public private partnerships, to enable major projects to proceed," it said.