You wouldn’t need a visa for a visit of 6 months or less with what you where sentenced to you would have to wait 10 years from conviction to be able to enter the uk for a visit. If you are still on the sex offenders register in New Zealand after 10 years they may still issue a green notice and then you also still maybe refused entry even after 10 years.

If you where to apply for a work visa then your offence as I understand it would be classed as spent after 6 years (of conviction) and you maybe eligible for that but you would need to get advice from a solicitor. If you where convicted before your 18th birthday that would change things quite abit. As it stands you can enter the Schengen area visa free for upto 90 days in any 180 days with no conviction questions (will change in 2022) but New Zealand may still issue a green notice.

I’m not an expert so take everything I’ve said with a pinch of salt. There’s a good immigration forum if you google uk immigration forum it’s the first one to come up there people asking similar questions on there if you ask your question on there they may have some kind of loophole for you.

https://www.immigrationboards.com/

Just been doing some research if your 2 year sentence was a community detention then that would be classes as community order in the uk and not the same as a prison sentence so you be ok to travel to the uk for upto 6 month with out any visa. If your sentence was above community detention then you would need to wait 10 years. I also seen that New Zealand sex offenders register is only for offences committed against children so as long as that’s not the case you will be fine in regard to the green notices.

If you travel to the uk for upto 6 months you won’t even have to see an immigration officer as because you are from New Zealand you can use the e gates at the airport.

Thank you so much for your help and advice.
So does that mean that a visa isn't required even though there is a criminal conviction? I thought that a visa had to be sought if there was a conviction 
Immigration law is such a minefield 

As far as I can tell as long as you’ve waited the correct amount of time all you need to travel is you New Zealand passport and no visa or waiver or anything else. The uk gov website says you may want to apply for a visa if you have a criminal conviction if I was in your situation I would avoid doing that as disclosing could cause problems with other stuff later on. It probably definitely worth speaking to a uk immigration lawyer just to double check before you come. People coming to the uk visa free are not routinely asked about criminal records and if you are as long as you’ve waited the correct time you should still get in.