Jamie Wilkinson made it 3-1 from set-piece play after Robertson challenged the Miramar keeper for the scorer to put it into an empty net.
Seventeen-year-old Zac Madson claimed his maiden goal in the league after two appearances off the bench.
Robertson said Miramar had a youthful look about them but some talented footballers among the lineup.
"They perhaps lacked a bit of experience but they were certainly a good side who put us under a little bit of pressure at times."
He said the Blues applied themselves in their first away game this season: "We defended well except for that one moment in the first half, with Ruben [Parker Hanks] not having much to do except for a save in the second half."
The Rovers were dangerous at the coalface and will return to the capital to play Wellington Olympic, who pipped Waterside Karori 3-2, this Saturday.
Building King Havelock North Wanderers lost 6-0 to Wellington United on Saturday. Stop Out beat Lower Hutt 2-1 while Wairarapa United drew 1-1 with Western Suburbs.