Infratil has reeled off a list of what it calls "unhelpful regulatory interactions" this year that have left the publicly listed infrastructure investor wary of both lawmakers and regulators.

The company, which is managed by Wellington-based investment bank HRL Morrison & Co, last week posted a 16 per cent gain in first-half profit, helped by the contribution from its 50.5 per cent-owned Trustpower, but also said it was a period when it sometimes felt wrong-footed by the state sector.

"At times it's difficult to retain confidence in law makers and regulators," chairman Mark Tume and chief executive Marko Bogoievski say in their first-half report.

Issues included the decisions of regional transport authorities in awarding contracts to operate public transport bus services, which have seen Infratil's NZ Bus unit cede ground to rivals.

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Last month the company said NZ Bus had concluded negotiations with Auckland Transport while negotiations in Wellington were ongoing, and overall it expected the business to shrink by about one third.

"NZ Bus and its people have been impacted by low-cost choices by regional transport authorities," Tume and Bogoievski said.

"Winning tenders appear to have been based on the operators paying lower wages."

In May, NZ Bus lost most of its bus contract with the Greater Wellington Regional Council to Masterton-based Tranzit Group, which the council said could offer the service cheaper, reducing its share of services to 28 per cent from 73 per cent.

Meanwhile, resource consent applications for a runway extension at Infratil's 66 percent-owned Wellington International Airport have been put on hold pending a court decision as to how the Civil Aviation Authority should interpret the regulations which guide its decisions.

Across the Tasman, Tume and Bogoievski say, the regulation of Australia's energy markets "continues to be in turmoil."

"On a number of occasions some of Tilt Renewables' operational wind farms were switched off because South Australian grid rules required the use of gas/coal fired plant," they said.

Tilt was created when Trustpower spun out its wind and solar generation facilities into a separately listed company in 2016. Question marks hang over its 2016 investment in US renewable energy company Longroad Energy, after US President Donald Trump "announced a policy to subsidise nuclear and coal fired generation."

The US Department of Energy has proposed that compensation to coal and nuclear plant operators include recognition of their contribution to the reliability of the national grid.

The proposal, which is being driven by US energy secretary Rick Perry, would extend the life of hundreds of aging coal and nuclear plants, critics say.

Special purpose funds associated with Infratil and Morrison & Co have been a significant partner to the government in recent years under various public-private partnership schemes including long-term concession arrangements to finance, build and maintain assets including prisons and schools and investment in the Pūhoi-to-Warkworth PPP motorway contract awarded last year.

Infratil said the Labour Party-led government "has something of a clean sheet on many policy areas" but in areas relevant to the company "their most clearly enunciated goals are to up the rate of infrastructure investment."

"However, it's less clear whether the government intends going it alone or working with private partners," it said.

Infratil shares last traded at 3.22, up 0.2 per cent on the day, and have gained 17 per cent this year.