The study team's leader, Associate Professor of Geology Phil Shane of Auckland University's School of Environment, said the drilling project had found numerous thin lava flows.
Watch: Auckland's volcano risk
The number of flows, which showed how the volcano had built up over time, had surprised the researchers, he said.
"I suppose it means it has been built up from smaller eruptions, but we can't yet say how much time is involved - that is one of the main aims of the investigation."
As the drill reached deeper, he expected to find where magma had interacted with water in the harbour after flowing out of the ground.
The EQC-funded study, a collaboration with Massey University, ultimately aims to find whether activity has been continuous or intermittent, how long episodes lasted, and whether the character of eruptions has changed.
"If we can get a detailed record of how these volcanoes grow, then we will know much better what to expect with future activity."
The information would help hazard and risk planners, such as Civil Defence, and improve models of future volcanic activity.
"While we can't predict the future, our research raises questions about if there was future activity in Auckland, how long would it last and could it be for longer periods rather than just months?"
Rangitoto, the largest of the 50 volcanoes that make up Auckland's volcanic field, might have been erupting for nearly 1000 years - a long time for that type of volcano, he said.
"That radically changes the perception of potential future hazards in the region and the implications for Auckland as New Zealand's largest city."
