NSW 6-figure abbreviated UTM grid reference to 1:25k map sheet. Handles AGD66 and MGA94 datums.

nsw_utm.js

//requires esri leaflet, leaflet, proj4js
//fiddle at https://jsfiddle.net/a8aLj0nk/13

var map = L.map('map').setView([-34.00, 151.00], 9);
var position = 0;
var marker;

L.esri.tiledMapLayer("http://mapsq.six.nsw.gov.au/arcgis/rest/services/sixmaps/LPITopoMap/MapServer", {
    detectRetina: true,
    reuseTiles: true
    
}).addTo(map);

var sheets = L.esri.featureLayer('http://mapsq.six.nsw.gov.au/arcgis/rest/services/sixmaps/Boundaries/MapServer/18', {
    style: function (feature) {
        return {
            color: 'grey',
            weight: 0.5,
            opacity: 0.5,
            fillOpacity: 0
        };
    }
}).addTo(map);


grRegEx = '^[0-9]{6,6}$';
var previousIds = [];

function processPoint(e){
console.log('Click: ' + e.latlng);
    sheet = document.getElementById('sheet_extents');
    var qr = sheets.query().nearby(e.latlng, 500);
    qr.count(function (error, count, response) {
        console.log('Found ' + count + ' features');
    });

    qr.ids(function (error, ids) {
        for (var j = 0; j < previousIds.length; j++) {
            sheets.resetStyle(previousIds[j]);
        }

        previousIds = ids;

        for (var i = 0; i < ids.length; i++) {
            console.log("Setting style for feature " + i);
            sheets.setFeatureStyle(ids[i], {
                color: 'blue',
                weight: 2,
                opacity: 0.5,
                fillOpacity: 0.1
            });
        }
    });

    qr.run(function (error, featureCollection, response) {
        for (var i = 0; i < featureCollection.features.length; i++) {
            tile = featureCollection.features[i];
            var poly = L.polygon(tile.geometry.coordinates);
            sheet.innerHTML = tile.properties.maptitle + '<br>' + tile.properties.mapnumber + '<br>';
            var projFrom = '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs';
            var projTo = '+proj=utm +zone=' + tile.properties.mgazone + ' +south +ellps=WGS84 +datum=WGS84 +units=m +no_defs';
            var proj66 = '+proj=utm +zone=' + tile.properties.mgazone + ' +south +ellps=aust_SA +towgs84=-117.808,-51.536,137.784,0.303,0.446,0.234,-0.29 +no_defs';
            var bounds = poly.getBounds();
            var neBoundUTM = proj4(projFrom, projTo).forward([bounds._northEast.lat, bounds._northEast.lng]);
            sheet.innerHTML += 'NE LatLng :' + [bounds._northEast.lat, bounds._northEast.lng] + '<br>';
            sheet.innerHTML += 'NE UTM :' + neBoundUTM + '<br>';
            var markpos = proj4(projFrom, projTo).forward([e.latlng.lng, e.latlng.lat]);
            if (typeof (marker) == 'object') {
                map.removeLayer(marker);
            }

            var gr = fromGridRef(document.getElementById('pointInput').value, neBoundUTM);
            console.log(gr);
            var pointgr;
            //get lat lng of the gr
            console.log(document.getElementById('is66').value);
            if (document.getElementById('is66').checked) {
                console.log('Transform using AGD66 datum');
                pointgr = proj4(projTo, projFrom).forward(gr);
            }
            else {
                pointgr = proj4(proj66, projFrom).forward(gr);
            }
            
            sheet.innerHTML += 'Point UTM: ' + gr + '<br>';
            sheet.innerHTML += 'Point ll: ' + pointgr + '<br>';
            console.log(': ' + pointgr);
            console.log(L.latLng([pointgr[1], pointgr[0]]));

            marker = new L.Marker(L.latLng([pointgr[1], pointgr[0]]), {
                draggable: true
            });
            console.log(marker);
            console.log(e);
            map.addLayer(marker);
        }
    });
}

var point;
map.on('mousedown', function (e) {
    point = e;
    processPoint(point);
});

//update when toggling checkbox

function fromGridRef(gridref, sheetref) {
    console.log('Input ref: ' + gridref);
    if (gridref.length > 1) {
        if (!gridref.match(grRegEx)) {
            alert("Not a valid grid ref: " + gridref);
        }
        var grE = +gridref.substring(0, 3);
        var grN = +gridref.substring(3, 6);
        var sE = Math.round(sheetref[0]);
        var sN = Math.round(sheetref[1]);
        console.log('sE = ' + sE);
        console.log('sN = ' + sN);


        //the 0,0 point of the sheet (for the 3 sig-figs of the GR we have)
        //might be mid-sheet, in which case we need to be -1 on an order of 
        //magnitude above

        var ssE = +('' + sE).substring(1, 4);
        if (grE > ssE) {
            console.log('3SF (E)' + ssE);
            sE = sE - 100000;
            console.log('Reduced sE to ' + sE);
        }
        var ssN = +('' + sN).substring(2, 5);
        if (grN > ssN) {
            console.log('3SF (N)' + ssN);
            sN = sN - 100000;
            console.log('Reduced sN to ' + sN);
        }

        var pointUTM_E = ('' + sE).substring(0, 1) + grE + '00';
        var pointUTM_N = ('' + sN).substring(0, 2) + grN + '00';
        return [+pointUTM_E, +pointUTM_N];
    }
}