New Zealand was asked to buy the Princess Ashika, several months before the Tongan ferry sank on 5 August, killing 74 people.
Documents obtained by Radio New Zealand under the Official Information Act show Tonga wanted New Zealand aid money to buy the ship as a stopgap while it waited the delivery of a new vessel.
The request was made in April this year. It stated that the Princess Ashika was seaworthy, having been surveyed in a dry dock in December 2008.
Tonga wanted the Princess Ashika to serve the outer islands, because it was concerned about the safety of an existing ship and a replacement was not due until 2011.
The documents reveal the Ministry of Foreign Affairs & Trade initially considered the purchase a good aid project.
However, officials later expressed concern about funding a 37-year-old vessel for just two years of service.
The documents also state the ministry felt that Tonga could afford to pay for the ship itself.
A Royal Commission of Inquiry into the sinking of the Princess Ashika continues in Tonga.
A quarter of Tongan fleet unseaworthy - Vi
Nuku'alofa's Port Authority manager says Tonga could face another marine disaster because a quarter of its shipping fleet is unseaworthy.
Commander Lupeti Vi told the commission his concerns about ship safety had been ignored by the Ministry of Transport many times.
Mr Vi says in his 10 years as Port Authority manager, the standards of vessel safety in Tonga have deteriorated.
He told the commission there should be thorough inspections and sign-offs by the owners and agencies such as the Port Authority, while crews should be fully qualified.
He says if there is no regulation to make sure these three actions work together, there will be another disaster.