They started irrigating in 2006 and already pay high costs to grow their crops for local and overseas markets.
"We pay thousands of dollars annually to the regional council for water consents, and that doesn't include our own hydrologists and infrastructure, and ongoing monitoring of our water take," said Phillip.
In addition, the power costs to apply the water total about $2.30 per millimetre.
Under Labour's proposed policy at even 1 cent per cubic metre, their costs would go up by about $30,000 a year, they said.
Such a hike rankles.
"We have invested heavily in technology to mitigate our environmental impact, including an independent environmental plan, extensive fencing of waterways and riparian strips," said Callum.
To keep costs under control and to get the best out of the crops requires efficient irrigation - no water goes to waste, he added.
They are concerned what the details of the policy might involve. Would they be taxed on their total consent, or just what they use? Would the tax include stock water? What is the tax meant to achieve?
This lack of detail concerns them, as does the rationale behind the policy, which they perceive as emotive rather than scientific.
"A lot of people think that all water problems are farmer-made. The lines are very blurred between public perception versus scientific fact. Will there one day be a tax on the amount of rain that falls on your property?" asked Leicester.
They currently employ five permanent staff whose wages are derived from the returns secured by irrigation, and those returns also put money in the pockets of other workers, from truck drivers to harvest crews, to McCains staff and further down the line.
"The crops we grow under irrigation are just the beginning of a supply chain including processing, logistics and marketing - creating employment for urban Hawke's Bay," said Phillip.
"Agriculture keeps Hawke's Bay growing - why would you want to put a tax on someone producing food?"
Although such a levy would bring significant increased costs, they would still survive, but it would mean less money in the coffers to pay for another part-time employee to work on continuing their planting and ongoing land management, said Callum.
In addition, there would be no alternative than to pass those costs onto the consumer, he added.
"If we have to pay for water it's really a levy on food and exports."
As a family, they say they want to create a sustainable business that will be in good shape, if not better, for future generations.
Margaret sums up the family's approach.
"We are the caretakers of the land - the income we make goes back into the farm - it's our duty to look after it."
