A black line was drawn diagonally through the adverts, with the contact details crossed out with a thick black felt pen. He had also heard a poster was pinned to a board at the marina saying Norsand was closed.
The Danish sailors said that on hearing the yard was closed four other boats that had intended heading to Whangarei had sailed on to Australia.
The sailors had also heard some boats had gone to Auckland for their boat repairs.
Mr Wilkinson said the news was unbelievable and while he was not sure who may have crossed out the advertisements he was sure of their intention.
"It appears to have been done by someone to gain a competitive advantage. It's sabotage," Mr Wilkinson told the Advocate.
"This was very methodically done ... it's very deliberate."
One of the people responsible for putting the marine guide together, Ray Roberts, said he was appalled to hear the publication had been tampered with.
"Each boat brings in about $50,000 and they spend that in the wider community, so the whole of Northland misses out as a result of this."
Mr Roberts said he supported Mr Wilkinson in his actions to find the culprit.
Mr Wilkinson said: "We have missed out on business but it also has a wider economic impact for Whangarei and Northland.
"The whole region misses out because those sailors spend thousands on hospitality, accommodation and tourism attractions and much more."
Northland Inc executive chairman Colin Mitten said Northland's marine services industry was about the best in the country and to hear of someone sabotaging that was extremely disappointing. The 300 yachts hosted in Northland generated millions of dollars for the region. The Whangarei Economic Development Group has put the annual earnings at $10million to $12million.