It’s clear that there are still species to find and describe in Hawaiʻi, even in a group as well-studied as Hawaiian plants. For example see this discovery relatively recently of a new species of lobelia on Oahu: https://phytokeys.pensoft.net/article/4678/.
The records of the Hawaiʻi Biological Survey for each year show that species are still being routinely discovered, almost all of which are animals. Fungi are not reported. The report is here: http://hbs.bishopmuseum.org/pubs-online/pdf/op129p1-2.pdf. Totals for new species discoveries of fauna and flora by year from the summary table in the report are in the table below.
Despite the resources available for Hawaiian insects, sources indicate that many species still remain to be discovered or described. Abundances are mostly unknown as illustrated by this statement in most of the 2015 DLNR SWAP factsheets on the various insect groups: “ABUNDANCE: Unknown. A lack of systematic surveys prevents any population estimate.”
For invertebrates the Bishop museum on their entomology webpage (https://www.bishopmuseum.org/entomology/) has this note: “The collection holds large but unknown numbers of endangered, threatened and extinct species. For virtually all arthropods, not enough research has been completed to even begin listing endangered or threatened species.”
For fungi about 400 species are currently known in Hawaiʻi. The number of species estimated on earth has varied greatly and is increasing over time. A 2017 paper estimates up to 3.8 M species with only about 120,000 currently known: https://www.asmscience.org/content/journal/microbiolspec/10.1128/microbiolspec.FUNK-0052-2016. A 2019 paper by Wu et al. estimates 12 M species: https://www.tandfonline.com/doi/full/10.1080/21501203.2019.1614106. Clearly there are many new species to be discovered everywhere.
And an even bigger area of potential new discovery is the microbiome.