Robertson (30) joins ex-Wellington star Jesse Ryder as a new name in the contract list, after Ryder announced his intentions to join the Otago squad earlier this month.
The remainder of the 13 contracts were largely predictable after the promotion of Hamish Rutherford and Neil Wagner to New Zealand Cricket contracts opened up the spots for Robertson and Ryder.
Ryder's arrival and Robertson's selection gives Johnson some confidence on the batting side, although the unluckiest players might have been in the bowling department.
''There were two or three contenders there who put their hand up and might have been unlucky,'' Johnson said.
Young seamer Blair Soper and veteran all-rounder Bradley Scott - who has returned from Northern Districts - were not out of the mix as the decision loomed for the final contract spot, which will be announced on August 9.
''There's still a contract to get out yet - but there's a number of contenders for that last contract.
''Over the next week, we've got to find out what's around and what we need to finalise a squad. But on the whole we're pretty happy with what we've got.''
Robertson gained his contract after getting regular game time last season, particularly in the domestic one-day competition, playing six games in the tournament. He also got on to the field for two twenty20 games and three Plunket Shield matches.
''He's batting cover in the middle there. He played a fair bit of one-day cricket last year, so he's a contender for that,'' Johnson said.
''It's about getting the right balance in terms of making sure you've got enough quicks as cover and understanding Hamish [Rutherford] and those other boys will be away a bit.''
Darren Broom is the main absentee from last season's 14-man list, as he has moved back to Christchurch.
The six major associations were able to offer initial contracts to between nine and 13 players. Northern Districts is the only association to have named fewer than 13, offering contracts to 11 players.
Otago Volts: Contracted players
Nick Beard, Michael Bracewell, Neil Broom, Ian Butler, Mark Craig, Derek De Boorder, Jacob Duffy, James McMillan, James Neesham, Aaron Redmond, Iain Robertson, Jesse Ryder, Sam Wells. Iain Robertson
