Greg Hunt, the environment minister, appears to have missed a self-imposed deadline to send a vessel to monitor the Japanese whaling fleet, which is homing in on the Southern Ocean whale sanctuary.

Hunt said the government was still committed to monitoring whaling, but that he would “have more to say on that in the coming days, we are just finalising the details”.

On 10 December, Hunt said he would make an announcement on the arrangements for monitoring within 10 days. This now looks unlikely to happen, with the environment minister understood to be in negotiations with cabinet colleagues over the logistics of the operation.

The Ocean Protector, a vessel that is custom built for the Southern Ocean, has seemingly been put out of Hunt’s reach, with the mid-year budget update assigning the boat to Operation Sovereign Borders, where it has been patrolling waters near Christmas Island for asylum seekers. It’s unclear which other vessels could be given the task.

In opposition, Hunt was a strident critic of Labor for failing to patrol Australia’s Antarctic waters territory to prevent the slaughter of whales. “We’ve got blood in the water and a blind eye in Canberra, it’s completely unacceptable,” he told reporters in February.

On Friday, Hunt said Australia will have a monitoring role “just as we promised”, revealing that the government had signed a statement, along with the US, New Zealand and Denmark, which condemns whaling and warns against “dangerous behaviour” on the seas.

But the Sea Shepherd anti-whaling group, which sent three of its vessels from Australia to confront the Japanese fleet on Wednesday, said Hunt is now running out of time.

“The government is already behind because the Japanese whaling fleet will be in the Southern Ocean whale sanctuary any day now,” Jeff Hansen, Sea Shepherd’s Australian director told Guardian Australia.

“The kill ships will be there, going after protected and endangered species. This is a pre-election promise that still hasn’t been met and I can’t understand what the delay is. They have known about this for a while.”

Hansen said the Sea Shepherd fleet could intercept the Japanese ship the Nisshin Maru as early as Christmas Day, adding that an Australian customs vessel would provide a deterrent to a “rampage” by the Nisshin Maru against anti-whaling activists.

Greens senator Peter Whish-Wilson told Guardian Australia that Hunt needs “conviction and guts” to immediately meet his election promise.

“I thought we had a different environment minister in Greg Hunt because he has been so vocal on the issue of whales,” he said. “He has got to have the courage to stand up for whales and to his own cabinet. A few more days takes us into Christmas when people are switching off. While Greg Hunt is tucking into his turkey, the Sea Shepherd will be doing the government’s job and engaging the Japanese fleet.

“Why can’t another vessel operate off Christmas Island, given the taxpayer has spent $150m on the Ocean Protector for the purpose of patrolling the Southern Ocean? If he doesn’t send a vessel, it’ll be because he knows it will have to confront and turn around the Japanese ships as the federal court has deemed whaling illegal in our waters. It’ll be a sad day if we trade off whales for sensitive trade deals with Japan.”