Infratil is upbeat about earnings in the coming year after reporting a sharp increase in its net surplus during the past 12 months.

The infrastructure investor reported a 26.3 per cent rise in full-year net surplus to $120 million for the financial year to March 31, in spite of flat earnings from its biggest investment - its stake in TrustPower - and being forced to write off $35 million from its European airports.

Higher than expected gains from its 50 per cent stake in Shell, now called Z Energy, and improved returns from its retail energy business in Australia, had contributed.

Infratil reported ebitdaf (earnings before interest, tax, depreciation, amortisation and financial instruments) up 27 per cent to $460 million.

For the current year it is forecasting ebitdaf up to $490 million, assuming gains for TrustPower, Wellington Airport, NZ Bus and Z Energy and allowing for a slight reduction from Infratil Energy Australia.

Chief executive Marko Bogoievski said the highlight of the past year had been the acquisition of Shell for $210 million, which contributed $116 million ($55 million in earnings and $61 million in asset revaluations) and the company was on the lookout for similar opportunities.

"We think there's a few more of those coming in the future."

The company believes the lid on government spending will mean increased private infrastructure opportunities.

Z's earnings for the past year were $157 million - $20 million higher than its final year under Shell ownership - and are forecast to grow to $170 million to $190 million.

Z's chief executive Mike Bennetts said growth would come from improved margins rather than volumes, which fell towards the end of the past year.

"We expect to have a reasonable amount of churn in our commercial activity. There are some customers who are lower value to us so we can churn those out and attract other customers who are a better match for us."

Earnings growth would also come from upgraded petrol station stores and investment in new or revamped sites. There were also gains to be made in its supply arrangements with the NZ Refining Company.

"We've got a track record of being able to add value that we account for in the refinery margin that's over and above what NZRC give back to us."

There was a range of responses to the Z rebrand since its launch last week but Bennetts said it had succeeded in informing the public it was a New Zealand-owned company.

Infratil Energy Australia's increased earnings were delivered despite intense competition in its main market in Victoria, major changes in the New South Wales market and work commissioning two power stations.

Earnings increased from $11 million to $55 million.

Wellington Airport passenger numbers remained stable as Tasman and regional traffic made up for a fall in main trunk traffic.

Earnings improved from $68 million to $72 million.

Infratil Airports Europe losses grew from $9 million to $11 million.

In response to questions on the $35 million write-off the company said it had been overly optimistic about the value of the business previously.

NZ Bus earnings grew from $29 million to $40 million as a result of higher fares and cost control.

Passenger growth in Auckland rose 6 per cent during the year.

The company said it would pay a final dividend of 4.25c a share, up 13 per cent on a year earlier.

Infratil shares closed up 4c yesterday at $1.94.