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Meterbox single phase, has mains going to isolation switch then through the meter then to a 63A MCB then to the MSB.
There is a Neutral and earth stud, the earth stud has two cables connected one seems to be a bonding cable for the meterbox and the other the earth for a Grid tied inverter.
There is a 25A MCB in the meterbox for the AC output of the Inverter, can the earth stud be used as the earth for the inverter circuit?
(25A MCB is feed from 63A MCB and the neutral is connected to the common neutral stud in the meterbox)

Maybe check this topic.

https://electricalforum.nz/viewtopic.php?f=5&p=696#p696

The advice DougP has pointed to relates to whether or not the netering point has been corecvtly set up as a 'switchboard", in which case it's the MSB and the switchboard it feeds must be a DB.

You didn't mention any MEN link at this point;
and with only those 2 wires on the E-stud seems
- there's no MEC going to an electrode from this enclosure; and
- there's no PEC for the "submain" to the inside swbd.
So on evidence so far it's not set up correctly as MSB.
In which case it's not permitted to connect the feed-in from the inverter system at that point; because all supplies must be fed in at a switchboard [2.3.3.1, also 5.3.1 of "4777.1"].

Even if the metering point is correctly set up as MSB; the installation of the supplementary supply (grid-tie inverter or other form of co-generation) isn't compliant.
The 25 A mcb can be used as "main switch inverter supply"; but must be marked as such, and the other one must be marked "main switch " [3000: 2.3.3.4 & 4777.1: 5.3.3]. Plus additional labelling requirements [4777.1: 5.5].
The earth of the inverter system can be connected to the MSB main earth connection, or to the installation's MEC. It can't be connected to anything else [3000: 5.5.2.1 & Fig 5.3].
And a single stud is not compliant as a n-bar; must have separate terminals for each incoming & outgoing circuit [2.9.4.3]

Probably wouldn't take a lot of work to make this installation compliant; by addressing these deficiencies

Ok Thanks for the answers, yes it's not the MSB the MSB is located inside. As the meter box is not a switchboard, the inverter will need to be connected to the MSB inside as per 2.3.3.1, there is however a label main switch inverter supply and other 4777 labelling. Also the neutral stud is actually a bar with 5 studs with incoming and outgoing main neutrals and meter neutrals connected. The earth connected is the meter box bonding. Sounds like to make it compliant need to either change the meter box to MSB and change MSB to DB board. OR run inverter AC output circuit inside directly to the MSB.