Regardless of his decision, the odds of it being judicially reviewed were ''quite high'', he said.
Dr Smith spent Friday and Saturday in the Fiordland and Mt Aspiring National Parks, spending Friday night on the Routeburn Track before inspecting the Hollyford airfield where Milford Dart Ltd proposed to deposit spoil from the 11.6km, $150 million tunnel.
On Saturday afternoon, he told the Otago Daily Times the trip ''felt very worthwhile''.
''You can read through hundreds and hundreds of pages of reports [but] it's not until you get on the ground, breathe the air, take in the landscape that you get a feel for the sorts of issues associated with it.
''It's probably the largest concession decision in the 25-year history of the department.''
The intention to grant the application was notified by Dr Smith's predecessor, Kate Wilkinson, in late 2011, attracting
1260 public submissions, heard in March and April last year.
Ms Wilkinson had intended to delegate the decision-making process. However, after the Conservation portfolio was given to Dr Smith in January, he decided to make the decision himself.
Under the Act, decisions of significance were to be made by the minister.
''To me, it would be something of a nonsense to say it was not a significant decision.''
Dr Smith said one of the key issues was with regard to the high threshold for activities proposed for national parks, since they were ''some of the highest valued conservation areas that the department is responsible for'' and some were World Heritage Areas. Another issue was the environmental effects of the proposal.
Having met the applicant in Christchurch last week, he will meet the hearings commissioner and the New Zealand Conservation Authority separately this week, but was not prepared to indicate when the decision - described as ''the largest and most difficult'' of his 24-year career - might be finalised.
''What I want to do after that process is be sure that I've got all of the information and advice that I need ...
I'm not going to make a decision until I've got all of the information.''
On Friday he had an ''informal discussion'' with representatives from Glenorchy's Stop the Tunnel group.
''I explained to them that the legal process under the National Parks Act and Conservation Act makes it quite plain that I would be acting unlawfully [to discuss the matter with them].
''My normal approach is to want to engage in discussions with people, but this is a very contentious issue - the chances of it being legally challenged is quite high.
''When there is such strong opposition and very large economic interests as well - both for and against - then the probability of a decision being judicially reviewed is quite high.
''I recognise there are very strong [feelings] on both sides ... I have no doubt whatever decision I make, it will be stridently criticised.''
