But neighbours say the machinery far exceeded the amount promised by the Department of Justice earlier in the week, and they feared it could have caused damage to native vegetation, with the area having received over 15 millimetres of rain in under two weeks.

The department initially planned to start core test drilling in October, but this was delayed due to wet soil and the discovery that threatened flora was "more widespread at the site than previously recorded" - a detail only revealed this month as part of a Right to Information request. Only one hole was drilled as a result.

Five or six holes will be drilled over the course of the next five or six days, according to the department, with the location of the holes in similar areas to those proposed in October.

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Since then, further evidence of the presence of a Tasmanian masked owl - listed as endangered - has been confirmed on the site.

Previous environmental consultant Ecotas - which has since resigned from the project - earlier provided advice to the department that the masked owl would "hunker down" in its borrow if present during drilling works, due to its nocturnal nature.

The works authority still lists the grey goshawk, green and gold frog, Tasmanian devil and wedge-tailed eagle as being threatened species either within, or near, the land. The authority is valid until July 31.

A Department of Justice spokesperson said the locations of the drill holes had been "carefully selected" to "manage the sensitive natural values" on the site.

She said they had enough environmental advice to begin drilling, when asked why it was commencing before carrying out a natural values assessment in spring.

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"The department considers it has obtained sufficient advice and information about the natural values on the site that enables the drilling works to be undertaken," the spokesperson said.

Neighbours, along with members of Westbury Region Against the Prison, gathered at the entrance and then entered the public land to observe the works, but were informed that they required approval of the project director due to the entire block being considered a work site.

WRAP president Linda Poulton said the use of heavy vehicles on the land could damage threatened flora, and she urged the department to wait until after spring to carry out the drilling works.