KiwiRail has been given $94.8 million from the Provincial Growth Fund to get the line back on track and build its freight capacity.
Capital projects and asset development chief operating officer David Gordon said survey of the tunnels had been completed and the bridge survey was due to be finished by the end of January.
He said consultants for bridge design work were expected to be hired this year and for the tunnel work early next year.
"We have begun the process of seeking contractor interest in the Northland work, but this is at an early stage. Our expectation is that those carrying out the work will include contractors based in the area."
Northland's railway line is mothballed north of Kauri and the whole line has been in a state of managed decline.
The rail line starts in Auckland and passes through Helensville and Whangārei to the dairy plant at Kauri, ending at a log-loading yard at Otiria, near Moerewa.
The last log train between Kauri and Otiria carrying logs of woodchip company Marusumi had its final run on August 31, 2016. The Dargaville branch line closed in October 2015.
KiwiRail officials briefed about 40 construction contractors in Whangārei in October about future work opportunities, requirements and necessary qualifications.
Gordon said the State-owned enterprise had ensured that, where possible, work on the rail line was broken into bite-sized pieces suitable for smaller contractors.
He said working in the rail corridor brought with it special safety requirements different from roads and other civil construction.
KiwiRail owns and maintains 270km of rail network in Northland and 3500km throughout New Zealand.
Breakdown of work from $94.8m PGF funding
• Track renewal: $53.1m
• Bridge replacement: $16.2m
• Drainage improvements: $9.5m
• Repairs to 13 tunnels: $7.3m
• Slope stabilisation: $4.7m
• Whangārei rail yard: $3.2m
• Vegetation control: $800,000
