The New Zealand Transport Agency has voiced concern over a low-cost technique for laying fibre-optic cable that the Government may need to rely on if it is to ensure its $1.35 billion ultra-fast broadband scheme can achieve its objectives within budget. The Economic Development Ministry has invited companies to carry out a trial of "micro-trenching", which involves laying fibre-optic cable in narrow, shallow slits cut in the road surface by circular saws, along with other techniques for deploying fibre. But Transport Agency group manager Ian Cox said in a submission to the ministry that shallow and micro-trenching could increase road maintenance costs and there was a risk fibre laid in micro-trenches could be easily damaged. It could also be unfair to, and increase costs for, other utility companies, which would have to avoid cutting the fibre, when digging up roads to carry out their own repairs. "It would be fair to say there is a fundamental level of concern about how new methodologies might be introduced when their ability to deliver under New Zealand circumstances is not understood," Mr Cox said. "No manager of a transport corridor is likely to accept practices that are of unproven efficacy and that may lead to increased long-term liabilities to all players." A study carried out for the Treasury in 2009 by consultant Murray Milner, who has since been appointed to the board of Crown Fibre Holdings, estimated it would cost between $4.5b and $10.4b to connect three-quarters of homes and businesses with fibre, depending on the technology used. He guessed 30 per cent to 40 per cent of homes could be connected cheaply using overhead cables costing $30 to $50 a metre. Dr Milner said micro-trenches would probably cost $50 to $70 a metre, and traditional trenching $120 to $600 a metre. The Institution of Professional Engineers said it feared Dr Milner had underestimated the costs of the ultrafast broadband scheme. "Both the proportion of overhead deployment and conventional trenching may also have been underestimated." It said in a joint submission to the Economic Development Ministry that local authorities needed to be open-minded, but the ministry also needed to demonstrate a "wider approach to New Zealand's economic interests, rather than a singular focus on the efficient deployment of broadband". Policy manager Tim Davin said the Transport Agency had not pulled any punches in its submission and the institution was trying to take a balanced view. It described shallow trenching technologies as "high risk", but said there might be situations where micro-trenching could be used without having a detrimental knock-on effect for other utilities. "Relative to the entire deployment, these opportunities may be small, but they are still important." The technology trials would include the "accelerated testing" of micro-trenching. The institution suggested fibre should be laid in a variety of roading materials in an indoor testing facility owned by the Transport Agency in Christchurch. There, the effect of decades of wear and tear from vehicle traffic could be simulated. Telecom network arm Chorus, Vector, TelstraClear, Wellington's Opto Networks and Waikato BOP Fibre consortium have expressed interest in carrying out "live" trials for the ministry.