Where is the evidence that having an upper house produces better law/is better for democracy?

The evidence is that NZ is almost unique getting rid of the necessary check and balance that an Upper House provides, and the protection it affords the sovereignty of parliament. Other democracies commonly have 2 house at both federal and state level. That is an enormous difference to the unbridled power our governments cabinet wields. In essence parliament is now a waste of time – Government MPs are purely there to provide the voting numbers. Opposition MPs have next to no role to play. There is a litany of legislation that would never have got through under a two House system because it breaches constitutional rights. The most dramatic was in 1975 with Muldoon bringing in NZ Super – a massive intergenerational transfer. I suggest you listen to the evidence I reference from Professor Jeremy Waldron as just one source.