I think there is a diconnect between the name on the tombstone and the name used in the records. It would seem that Isaiah is Richard in the BDM records.
The Symonds Street Cemetery Anglican Sexton's list has John and Isaiah Robinson at Block C 156 and C 157.
According to the late 1950's tombstone transcriptions John Willis Robinson died 24 May 1863 aged 83 years, Isaiah Robinson died 24 May 1881 aged 18 years.
The St Paul's register has John Robinson buried 9 November 1870 aged 50 years of Newton Road. This is the father of Isaiah/Richard.
The Death certificate index for 1881 has Richard Robinson instead of Isaiah and the St Paul's register has Richard Robinson buried 25 May 1881 aged 16 years of Newton Road.
The Auckland Star newspaper, on 24 May 1881, had this article:
Death - Robinson - on the 24th instant, at Newton, Richard, youngest son of the late John Robinson, 16 years. The funeral will take place tomorrow (Wednesday), 25th instant, at 2:30 pm. Friends accept this intimation.
Poss Birth Reg: 1865/6200 Robinson Richard
Death Registration: 1881/218 Robinson Richard 16Y
Death Reg: 1870/2096 Robinson John NR
So if this Richard is Isaiah, he was not born at sea.