The majority of the external costs - $612,830 - was spent on legal fees with Mai Chen of Chen Palmer representing the council during the inquiry.
The council also spent $444,251 on its own investigation to determine whether the contamination was in part or wholly the result of resource consent conditions not being met.
The outcome of this investigation saw the regional council drop its two charges against the Hastings District Council for unlawful water takes.
The district council had conceded the breach of resource consent conditions and agreed to not contest two infringement notices, resulting in two fines of $500 apiece.
The amount spent on the investigation included $211,977 in external costs - with $100,000 spent on external laboratory testing, and $20,000 on legal advice from Jonathan Krebbs for the investigation for prosecution.
Mr Maxwell wrote the past period had seen further significant costs "given the nearly two weeks of hearing time".
"We have actively managed the attending Counsel so that a junior Counsel was in
attendance during sessions that were not directly related to HBRC," he stated.
"Nonetheless there remains a significant cost, not all of which has been fully invoiced yet."
While there would be further costs associated with the second stage of the inquiry, it was thought these would be "significantly less" than for stage one.
The council had "tentatively" engaged a locally based planning consultancy to assist with the second stage.
"We anticipate that the costs associated with stage two will be substantially less than stage one as we see this process being led by the planners," he stated.
Mr Maxwell wrote they understood the inquiry panel would, or had already, sought permission from the Government to extend the reporting dates to allow stage one to be reported in May, and stage two toward the end of the year.