Last year, Infratil said NZ Bus's loss of certain public transport contracts meant it would shrink to about two-thirds its original scale, with 75 per cent of the business in Auckland and 25 per cent in Wellington.
In May 2017, 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.
NZ Bus contributed $17.9m to Infratil's $291m in first-half underlying earnings before interest, tax, depreciation, amortisation and financial adjustments, a 28 per cent drop on the year-earlier period which it said at the time reflected the loss of South Auckland services and costs to transition to a new operating model.
NZ Bus was valued at $179m as at September 30, down from $191m a year earlier, while Infratil's total assets were $2.79b.
Infratil has retained UBS New Zealand as an adviser for the review.
Infratil shares fell 2.3 per cent to $3.03.