It states that the group left Indonesia on May 5 and travelled for 12 days before encountering Australian officials.
After confirming they were not attempting to enter Australia, the group were allowed to continue, the letter states.
A short time later the asylum-seekers were turned around by Australian authorities, after being transferred to two smaller boats, the group's letter states.
When a boat hits rocks near an Indonesia Island, "we jump over the boat to sea and swim more than one and a half hours [to] reach island".
Immigration Minister Michael Woodhouse has been contacted for comment.
His spokeswoman earlier told Radio New Zealand that the Government had not received the letter, and the group could not claim asylum without being in New Zealand.
Prime Minister John Key has previously said he had been advised that the boat heading for New Zealand had a credible chance of making it.
Last week Amnesty International's global secretary general Salil Shetty travelled to New Zealand and asked Mr Key to double the country's refugee quota, currently at about 750 per year.
There are currently about 52 million refugees around the world - numbers not seen since World War II.