Compare with Lemonade lemon hybrid.
http://www.citrusvariety.ucr.edu/citrus/lemonade.html

Here is a link that might be useful: Lemonade lemon hybrid

Have you checked to see if the branch with the 'new' fruit might perhaps come from the root stock below the graft near the ground?

Brother has one of these at his new place. Same as yours on the inside but bumpier like some mandarins.

I'm with augment; see if it isn't a "rough lemon" - it is likely the rootstock sprouted and that got large enough to fruit.

So … you both realize that you replied to a 5-year old post?

Depending on how I enter into the Houzz forums you can see old posts displayed so it is easy to make this mistake if you are not paying attention.

Does anyone have an answer on this? Also living in Perth Australia with this same fruit.

I live in Adelaide. We had a traditional lemon tree that had to be cut down due to termites. The stump regrew eventually and when we first got fruit back, they looked exactly like the ones pictured here.

Turns out a neighbour has a mandarin tree and our lemon was cross-pollinated. The next season, we had a combination of regular lemons and the hybridised variety. Fast forward a few seasons and we were back to regular lemons.

@ Nicole, yours would likely be a grafted tree and the rootstock grew and produced fruits, after the top part was cut back. Then later some of the grafted part regrew, so you got a combination of rootstock and grafted tree producing fruits at the same time.

FYI, a citrus tree will not produce hybrid fruits from cross pollination, only hybrid seeds, which would have to be planted and grown out to get the hybrid fruit.

In fact no fruiting plant I can think of will produce a hybrid fruit from cross pollination.

I nade lemon curd & lemon slice with some I had given me.

They were amazing.

Just made another batch out if lemons & not even clise to blend or look

Do you know wher I can buy in south Audtralua.

Don't know how you did it but don't stop

Going to tty lemonade if I can get more of them

I live in the pilbara WA and we’ve just found the same thing out in our horse yard, petty much couldnt find anything online other than this post we call them mandalems

I am eating one right now from my brothers house. lol