Tongan Transport Minister Paul Karalus has resigned, six days after the sinking of the Princess Ashika.

At least 149 people were on board the Tongan inter-island ferry when it capsized last Wednesday, 86km northeast of the Tongan capital of Nuku'alofa. Two bodies and 54 survivors have been found, while 93 people are presumed drowned after being trapped in the vessel.

They include 33 women and 10 children who were sleeping on the lower indoor decks.

Mr Karalus announced his resignation today, saying it was necessary to enable a Royal Commission of Inquiry into the sinking.

"My ministry is the one that is the subject of the Royal Commission so one, from a legal perspective if no other, has to step aside to allow the process," he told Radio New Zealand.

"One cannot be judge and judged at the same time."

Mr Karalus said his resignation was not an admission of responsibility and maintained that the Princess Ashika was seaworthy.

"We carried out our duties with due care and diligence," he said.

New Zealand navy divers could know late tonight whether the object attached to a rope spotted in Tongan waters is the Princess Ashika.

Lieutenant Commander Andrew McMillan told NZPA a Tongan Defence Service patrol craft returning to Nuku'alofa yesterday spotted a rope in the water and further investigation revealed it was "attached to something down below".

"The rope was found in the vicinity of the oil slick that we've been looking at and the last known position of the locator beacon," he said.

"We've spoken to the port company and it has been determined that the rope is of the type that was used by the Princess Ashika so it could be associated with the vessel."

The rope was less than 455m from the slick and the beacon position but Lt Cdr McMillan said it could not be assumed it was the wreckage.

"I've got to stress that this is only another indicator but all of these things put together make this a priority site for us to go and inspect," he said.

New Zealand navy divers aboard a Tongan vessel were on their way to the site late this afternoon and would possibly dive down the rope, depending on the weather.

Strong winds and high seas earlier today had prevented any searches by divers or the autonomous underwater vehicle -- "which is like a torpedo which flies around under the water scanning the bottom".

It was unlikely any diving would be possible today "but we're sending a team out there to try".

"They can dive in the dark and if they do get there at a reasonable hour they'll have a look," Lt Cdr McMillan said.

"But probably one of the main reasons is the ship will be able to go to anchor overnight near one of the islands and the weather is a little bit calmer in the early mornings here, so they may be able to get out first thing in the morning and do a dive."

Lt Cdr McMillan had mixed emotions about the possibility it was the Princess Ashika at the bottom of the rope; it would be great to find it but the 100m depth the object was at was 40m deeper than the New Zealand and Australian divers in Tonga for the search could go to.

Some organisations worldwide were capable of such a deep dive but if the Princess Ashika was found in more than 60m of water, "that would be a matter for the Kingdom of Tonga to deal with".

Tongan police commander Chris Kelley would not be drawn on the likelihood the object was the ferry.

"It's a little bit of evidence but we're being very careful not to speculate on what it means," he told NZPA.