The work includes enlarging both the theatre and retail area.
Mr McManus said the time was right for the development, following the opening of the $14 million Te Kongahu Museum of Waitangi in February.
"Our visitor numbers are growing rapidly, especially now the museum is open and receiving such positive feedback. The current visitor centre is simply too small to cope with the numbers, especially in summer, so the extension is much needed and overdue," he said.
The developments at Waitangi could be taken as a sign of the times for the Bay of Islands' visitor industry in general.
Northland Inc's tourism and promotions head Paul Davis said visitor numbers to the area had increased 4.5 per cent in the year until April.
"Anecdotal feedback from key tourism sector personnel has reinforced the significance and value the Waitangi Museum will create for Northland's tourism sector and economy," Mr Davis said.
Recovering from a wet, drab start to summer, the sector has also seen month on month growth in "guest nights" - people hiring a bed per night - up 5.8 per cent in February compared with February the year before, 11.5 per cent in March and more than 22 per cent in April.
Meanwhile, 55 per cent of visitors to Waitangi are international and 45 per cent are from within New Zealand, those figures mirroring the general tourism profile in the district. Other than walk-in independent visitors and land or cruise ship tourists, the next main user of the Waitangi Treaty Grounds is the educational sector.
Te Kongahu Museum of Waitangi was co-winner of the New Zealand Museum's 2016 Best Museum Project Award. Kaikohe firm Henwood Construction has the visitor centre building contract, as it did the museum project.
Work should be completed by October.
The trust offers a day pass that includes admission to the museum and grounds, a guided tour and a cultural performance. International visitors pay $40, New Zealanders pay half that at $20, and children up to age 17 can enter free but must be accompanied by a parent or caregiver.