Bethune, 45, hopes to receive a suspended sentence, according to his wife Sharyn, in Auckland, but has said he is "very nervous".
"I will accept the verdict, but I do hope to go home soon," Peter Bethune, a former member of the Sea Shepherd Conservation Society, told Kyodo News agency during an interview at Tokyo Detention House.
He pleaded guilty last month to charges relating to the illegal boarding of a Japanese whaling ship, but not guilty to assault, after being held in custody since February, when he boarded the Japanese whaling fleet's security ship the Shonan Maru II during its annual trip south.
Sharyn Bethune has said she hopes her husband will be able to return to New Zealand - and his daughters Danielle and Alycia - as early as Saturday morning.
If he receives a jail sentence, his family will fly to Japan to see him.
A Ministry of Foreign Affairs spokesman told NZPA a New Zealand government official would attend the sentencing.
Bethune was captain of the US-based environmentalist group's futuristic powerboat, the Ady Gil - his former record-setting trimaran Earthrace - which sank after an earlier collision with the Shonan Maru II.
He tried to serve a citizen's arrest warrant to the captain of the Shonan Maru II over the sinking of his vessel and the attempted murder of him and his crew, but was captured.
"I deeply regret" if someone was injured in the Antarctic Ocean, he said ahead of his ruling tomorrow.
"I have nothing against Japanese."
At his trial in Tokyo, Bethune pleaded guilty to charges including trespassing, vandalism and holding a knife, which he used to cut netting as he climbed onto the ship from a jet ski, but he has denied the assault charge.
He was also accused of causing chemical burns to the face of a whaler with a rancid butter stink bomb and four other charges.
"What I was doing was standing up for New Zealand and Australia," he told Kyodo.
He stressed that it was "deeply offensive" for the two countries to see the Japanese hunting whales specifically in the Antarctic Ocean, and that he "wouldn't mind if it is Northern Pacific".
The Sea Shepherd group has said Bethune would no longer join its actions after taking a bow and arrows on the latest high-seas campaign, contrary to Sea Shepherd's stance of "aggressive but non-violent direct action".
