Whanganui people have a rare chance to see the New Zealand mainland's rarest native tree in full flower at Virginia Lake.

The Bartlett's rata tree is about 100m from the lake's twin bridges. It's in full bloom, Whanganui ecologist Colin Ogle said.

"It's by far the largest flowering we've ever seen on it. It's totally covered."

He wanted to let people know, so they could go and see.

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"If they go in the next week or two they might catch it flowering. It's something they will almost certainly never see in the wild."

There are at least 10 species of rata in New Zealand. Bartlett's rata was discovered by school teacher John Bartlett in a forest remnant in Northland in 1975

In 1992 there were 34 trees left in the wild. By 2003 there were 25 left and last year there were only 14.

The trees in the wild are mainly on private land. They are at risk from browsing possums, and from fire. They aren't setting much seed, because there aren't enough nectar-eating birds flying from tree to tree. What seed they do set has limited genetic variety.

Mr Ogle bought his Bartlett's rata plant from Graeme Platt's Auckland nursery, in about 1980. He grew it in pots, until donating it to the Whanganui reserve.

It was planted there around 2000, near a wet area. It has only had a few flowers since.