KiwiRail has set a date for restoring passenger services to Onehunga, but Auckland Regional Council chairman Mike Lee is unhappy that it is not until September 19.

The Government rail company has acknowledged "some slippage" on preparations to re-open the 3.6km branch line between Penrose and Onehunga, but says it needs time to complete all necessary tasks without compromising safety or reliability.

Spokeswoman Jenni Austin said the date for restoring passenger services after 37 years had been agreed between KiwiRail, the Auckland Regional Transport Authority and train operator Veolia Transport.

Enough time was needed for the authority to finish building three stations on the line and for KiwiRail to complete signalling and other infrastructure before train drivers could familiarise themselves with the setup.

Auckland City granted a minor modification to the corridor last month to allow KiwiRail to lay tracks along a small spur to an 0.8ha site which the regional council bought more than two years ago for a terminus station at the southern end of Onehunga Mall.

But KiwiRail is still waiting for approvals of an outline plan and an assessment of environmental impact before it can start building platforms.

Ms Austin said those approvals were expected this month, and the parties were planning a community celebration and education campaign leading to the resumption of passenger services to Onehunga on September 19.

Her assurances did not allay fears of Mr Lee, who has spent years battling for the branch line to be reopened.

"I find it very disappointing," he said of the latest delay, from a previous target date of July 1.

"Apparently they've got other work to do but the recommissioning of the Onehunga branch line was signed off by [Labour Finance Minister] Michael Cullen in April 2007 and they've had long enough to do it.

The regional transport authority is preparing to rejig services on the southern railway line to allow two trains an hour from Onehunga to Britomart at peak times and hourly departures between times.