The New Zealand Plant Conservation Network

I have gathered seeds from this tree and I wonder the best way to successfully plant them, please?

Not sure sorry, the New Zealand Plant Conservation Network focuses on native plant species. Perhaps contact you local garden centre, or google for germination tips for this species

As t happens, it is good that you posted this question here, Laura, as that species is a weed in NZ, and therefore shouldn't be propagated. If you look up the Weedbusters website, it explains why.

On that website, it offers the following alternatives: Try the native koromiko (Hebe salicifolia) or kawakawa (Macropiper excelsum), or the non-native correa species or fuchsia hybrids. Your local garden centre will be able to recommend other non-weedy alternatives that will grow well in your area.

If you are after lilac pea flowers, some species or forms of our native brooms (Carmichaelia) are particularly floriferous. As an alternative, an Australian shrub from the pea family, called Eutaxia, has a similar flower form (in orange) and is not environmentally problematic.