"If they can do that, then we figure they're ready to go," Dr Gartrell said.
Wildlife experts also had to ensure the birds' bodyweight and protein levels were adequate and that their coats, weakened by oil, could properly resist water again. Readjusting from a lifestyle of regular fish smoothies and round-the-clock care would be a shock "but the good thing about little blue penguins is that as soon they see the ocean, they'll be off - and they won't be looking back".
The shags, penned in pools covered by netting, had been preening for weeks. "You can see them flapping their wings and they're very much good to go."
Dr Gartrell hoped that by the end of the month all 406 birds would be back in their natural habitats.
The last birds to be released were likely to be the few penguins brought in over recent days and the 60 endangered New Zealand dotterels, which could be disturbed by ongoing beach clean-ups.
Dr Gartrell said the first release would be an emotional moment.
"It really will be a turning point in the whole process that we'd all love to see.
"We weren't able to think about releasing any birds until all of the oil is off and it's a huge relief that we now can."
Wildlife experts were expected to stay for six more weeks at the base, which could be re-established to cope with another major event.
"We'll have some sort of presence here as long as the ship is on the reef."
The wildlife death toll remained unknown, Dr Gartrell said, but the number could pass 20,000.
That included a generation of unborn penguin chicks sacrificed to save their parents and a large number of little diving petrels.
"We had 800 of those birds through the post-mortem tent and of the five that came in alive, none of them made it through the wash tents.
"They're deep ocean birds, so most ofthose that died would have most likely drowned before they could reach the shore to get help.
"It's certainly been a major environmental disaster for New Zealand and it's certainly had an impact on seabird life here in the Bay of Plenty - it will take time to see just how big that will be."
Meanwhile, the crane barge Sea Tow 60 has moored alongside the Rena before the next phase of the salvage operation - removing the 1280 containers. Another, larger crane barge, Smit Borneo, is on its way from Singapore to assist.
"Each set of containers will present its own unique challenges," Maritime New Zealand salvage unit manager Arthur Jobard said.
"This means it is impossible to predict exactly how long it will take to safely remove all of the containers - but realistically, it is likely to take several months of patient work."
He said the Rena's fragile state meant it could break up before all containers were offloaded.
The outlook
* Seven months - the time it could take to offload the Rena's containers.
* Six weeks - the time the oiled wildlife centre is expected to remain in operation.
* Six months - the period the Government has to decide whether it will prosecute the Rena's owners.
* One year - the timeframe of a long-term environmental recovery plan.
* Several years - the expected timeframe of ongoing environmental monitoring.
* Unknown - the time it will take to mop up oil remnants still on board the Rena and on beaches.
