"Preliminary assessments indicate that a mechanical failure caused the loss of power, which then resulted in a loss of steering," Mr Grogan said.
The container vessel was leaving the harbour after having come in to refuel on its way from Auckland to Noumea.
Mr Grogan said the harbour passage was not a difficult route to pilot.
"I wouldn't have thought it was a difficult harbour to navigate. This sort of thing is not common," he said.
But Mr Grogan said the loss of power on board was not considered anything out of the ordinary.
In the past 18 months, he said, another vessel had also lost power in the harbour.
"While mechanical failure on ships isn't a common problem, these things happen from time to time. Our harbourmasters are well-trained to deal with situations of this type.
"The key thing is it lost power. When that happens, the vessel is at the whim of the currents," he said.
Tauranga's Mayor Stuart Crosby was demanding quick answers on whether industry-wide problems were behind two ships running aground off Tauranga within the space of one month.
"This must be of concern to our community. Is there an underlying problem?" Mr Crosby asked.
He said the community needed to see the preliminary findings into both groundings as soon as possible so that people could have confidence in the navigation of ships in and around the harbour - and not in six to 12 months' time.
Mr Crosby understood that a ship's equivalent to an aircraft's black box had been taken off the Rena very soon after it grounded, with the data sent to Australia to be analysed.
He said it came down to the quality and maintenance of the vessels operating off New Zealand's coast and said it was just luck that yesterday's grounding happened in calm weather.
"In our case, clearly there is no acceptable risk.
"Questions need to be asked and answers quickly given," he said.
Graeme Marshall, commercial manager at Port of Tauranga, said the ship ran aground after bunkering in the port but insisted the maritime mishap was managed well and residents could have faith in the management of vessels coming and going from the harbour.
"I think there should be a high degree of confidence. We have a lot of ships that bunker here. We have 3000 movements a year and very few incidents," he said.
"I can say that oil booms were placed on standby immediately. The vessel came off within 19 minutes."
Mr Grogan said the Schelde Trader grounding was not related to the Rena and would have run aground regardless of other situations.
"This is one of those things.
"If the Rena wasn't on the rocks these kinds of things still happen," he said.
Maritime New Zealand will look into the crash, spokesperson James Sygrove said.
"As far as we understand it, the ship hit the north rock.
"Maritime New Zealand is currently gathering more information before we decide to investigate this further," he said.
